drm/i915: Short-circuit i915_gem_wait_for_idle() if already idle

drm/i915: Short-circuit i915_gem_wait_for_idle() if already idle

If the device is asleep (no GT wakeref), we know the GPU is already idle.
If we add an early return, we can avoid touching registers and checking
hw state outside of the assumed GT wakelock. This prevents causing such
errors whilst debugging:

[ 2613.401647] RPM wakelock ref not held during HW access
[ 2613.401684] ------------[ cut here ]------------
[ 2613.401720] WARNING: CPU: 5 PID: 7739 at drivers/gpu/drm/i915/intel_drv.h:1787 gen6_read32+0x21f/0x2b0 [i915]
[ 2613.401731] Modules linked in: snd_hda_intel i915 vgem snd_hda_codec_hdmi x86_pkg_temp_thermal intel_powerclamp snd_hda_codec_realtek coretemp snd_hda_codec_generic crct10dif_pclmul crc32_pclmul ghash_clmulni_intel snd_hda_codec snd_hwdep snd_hda_core snd_pcm r8169 mii mei_me lpc_ich mei prime_numbers [last unloaded: i915]
[ 2613.401823] CPU: 5 PID: 7739 Comm: drv_missed_irq Tainted: G     U          4.12.0-rc2-CI-CI_DRM_421+ #1
[ 2613.401825] Hardware name: MSI MS-7924/Z97M-G43(MS-7924), BIOS V1.12 02/15/2016
[ 2613.401840] task: ffff880409e3a740 task.stack: ffffc900084dc000
[ 2613.401861] RIP: 0010:gen6_read32+0x21f/0x2b0 [i915]
[ 2613.401863] RSP: 0018:ffffc900084dfce8 EFLAGS: 00010292
[ 2613.401869] RAX: 000000000000002a RBX: ffff8804016a8000 RCX: 0000000000000006
[ 2613.401871] RDX: 0000000000000006 RSI: ffffffff81cbf2d9 RDI: ffffffff81c9e3a7
[ 2613.401874] RBP: ffffc900084dfd18 R08: ffff880409e3afc8 R09: 0000000000000000
[ 2613.401877] R10: 000000008a1c483f R11: 0000000000000000 R12: 000000000000209c
[ 2613.401879] R13: 0000000000000001 R14: ffff8804016a8000 R15: ffff8804016ac150
[ 2613.401882] FS:  00007f39ef3dd8c0(0000) GS:ffff88041fb40000(0000) knlGS:0000000000000000
[ 2613.401885] CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
[ 2613.401887] CR2: 00000000023717c8 CR3: 00000002e7b34000 CR4: 00000000001406e0
[ 2613.401889] Call Trace:
[ 2613.401912]  intel_engine_is_idle+0x76/0x90 [i915]
[ 2613.401931]  i915_gem_wait_for_idle+0xe6/0x1e0 [i915]
[ 2613.401951]  fault_irq_set+0x40/0x90 [i915]
[ 2613.401970]  i915_ring_test_irq_set+0x42/0x50 [i915]
[ 2613.401976]  simple_attr_write+0xc7/0xe0
[ 2613.401981]  full_proxy_write+0x4f/0x70
[ 2613.401987]  __vfs_write+0x23/0x120
[ 2613.401992]  ? rcu_read_lock_sched_held+0x75/0x80
[ 2613.401996]  ? rcu_sync_lockdep_assert+0x2a/0x50
[ 2613.401999]  ? __sb_start_write+0xfa/0x1f0
[ 2613.402004]  vfs_write+0xc5/0x1d0
[ 2613.402008]  ? trace_hardirqs_on_caller+0xe7/0x1c0
[ 2613.402013]  SyS_write+0x44/0xb0
[ 2613.402020]  entry_SYSCALL_64_fastpath+0x1c/0xb1
[ 2613.402022] RIP: 0033:0x7f39eded6670
[ 2613.402025] RSP: 002b:00007fffdcdcb1a8 EFLAGS: 00000246 ORIG_RAX: 0000000000000001
[ 2613.402030] RAX: ffffffffffffffda RBX: ffffffff81470203 RCX: 00007f39eded6670
[ 2613.402033] RDX: 0000000000000001 RSI: 000000000041bc33 RDI: 0000000000000006
[ 2613.402036] RBP: ffffc900084dff88 R08: 00007f39ef3dd8c0 R09: 0000000000000001
[ 2613.402038] R10: 0000000000000000 R11: 0000000000000246 R12: 000000000041bc33
[ 2613.402041] R13: 0000000000000006 R14: 0000000000000000 R15: 0000000000000000
[ 2613.402046]  ? __this_cpu_preempt_check+0x13/0x20
[ 2613.402052] Code: 01 9b fa e0 0f ff e9 28 fe ff ff 80 3d 6a dd 0e 00 00 0f 85 29 fe ff ff 48 c7 c7 48 19 29 a0 c6 05 56 dd 0e 00 01 e8 da 9a fa e0 <0f> ff e9 0f fe ff ff b9 01 00 00 00 ba 01 00 00 00 44 89 e6 48
[ 2613.402199] ---[ end trace 31f0cfa93ab632bf ]---

Fixes: bb6c9a750cd0 ("drm/i915: Wait for all engines to be idle as part of i915_gem_wait_for_idle()")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170530121334.17364-1-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
(cherry picked from commit 863e9fde1a7061dad09bb299c65bed5f1ccb44ff)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>