drm/i915/display: Fix NULL-crtc deref in calc_min_cdclk()

drm/i915/display: Fix NULL-crtc deref in calc_min_cdclk()

[   23.419442] BUG: KASAN: null-ptr-deref in intel_plane_calc_min_cdclk+0x82/0x440 [i915]
[   23.419527] Read of size 4 at addr 00000000000000f8 by task insmod/735
[   23.419578]
[   23.419644] CPU: 2 PID: 735 Comm: insmod Not tainted 5.5.0+ #114
[   23.419716] Hardware name: ��������������������������������� ���������������������������������/���������������������������������, BIOS RYBDWi35.86A.0246.2
[   23.419793] Call Trace:
[   23.419864]  dump_stack+0xef/0x16e
[   23.419927]  __kasan_report.cold+0x60/0x90
[   23.420157]  ? intel_plane_calc_min_cdclk+0x82/0x440 [i915]
[   23.420397]  intel_plane_calc_min_cdclk+0x82/0x440 [i915]
[   23.420630]  intel_atomic_check+0x455f/0x65a0 [i915]
[   23.420708]  ? mark_held_locks+0x90/0x90
[   23.420929]  ? intel_crtc_duplicate_state+0x2e/0x1b0 [i915]
[   23.421172]  ? intel_plane_duplicate_state+0x2d/0xc0 [i915]
[   23.421239]  ? __drm_dbg+0xa4/0x120
[   23.421303]  ? __kasan_kmalloc.constprop.0+0xc2/0xd0
[   23.421355]  ? __kmalloc_track_caller+0x23a/0x320
[   23.421602]  ? intel_calc_active_pipes+0x1c0/0x1c0 [i915]
[   23.421852]  sanitize_watermarks+0x220/0x510 [i915]
[   23.422092]  ? intel_atomic_check+0x65a0/0x65a0 [i915]
[   23.422164]  ? drm_modeset_unlock_all+0x88/0x130
[   23.422402]  intel_modeset_init+0x1b76/0x3c90 [i915]
[   23.422647]  ? intel_finish_reset+0x2d0/0x2d0 [i915]
[   23.422851]  ? intel_irq_install+0x12c/0x210 [i915]
[   23.423076]  i915_driver_probe+0x13e7/0x2930 [i915]

v2: No crtc is implied by an invisible plane, so the extra !crtc check
is redundant.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200203133824.198872-1-chris@chris-wilson.co.uk