drm/i915/guc: Don't try to create log runtime if there is no log

drm/i915/guc: Don't try to create log runtime if there is no log

In case of GuC initialization failure we may continue with driver
load, but we wrongly assume that GuC is fully functional. This
leads to the BUG as we attempt to access non-existing log vma.

[26386.121085] BUG: unable to handle kernel NULL pointer dereference at 00000000000000a0
[26386.121225] IP: guc_log_runtime_create+0x23/0xe0 [i915]
[26386.121763] Call Trace:
[26386.121870]  guc_log_late_setup+0xfd/0x140 [i915]
[26386.121969]  i915_driver_load+0x7ab/0x1730 [i915]
[26386.122069]  i915_pci_probe+0x2d/0x90 [i915]
[26386.122089]  pci_device_probe+0x9c/0x120
[26386.122107]  driver_probe_device+0x2a9/0x490
[26386.122126]  __driver_attach+0xd9/0xe0
[26386.122143]  ? driver_probe_device+0x490/0x490
[26386.122158]  bus_for_each_dev+0x57/0x90
[26386.122175]  bus_add_driver+0x1eb/0x260
[26386.122190]  ? 0xffffffffa069a000
[26386.122206]  driver_register+0x52/0xc0
[26386.122220]  ? 0xffffffffa069a000
[26386.122234]  do_one_initcall+0x39/0x170
[26386.122252]  ? kmem_cache_alloc_trace+0x1fd/0x2e0
[26386.122273]  do_init_module+0x56/0x1ec
[26386.122289]  load_module+0x219e/0x2550
[26386.122309]  ? vfs_read+0x121/0x140
[26386.122331]  ? SyS_finit_module+0xa5/0xe0
[26386.122346]  SyS_finit_module+0xa5/0xe0
[26386.122371]  entry_SYSCALL_64_fastpath+0x22/0x8f

Signed-off-by: Michal Wajdeczko <michal.wajdeczko@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Daniele Ceraolo Spurio <daniele.ceraolospurio@intel.com>
Cc: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180131173241.19704-4-michal.wajdeczko@intel.com
Reviewed-by: Sagar Arun Kamble <sagar.a.kamble@intel.com>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>