drm/i915: Ensure state->crtc is non-NULL for plane updates

When disabling a plane, it is legal to pass crtc = NULL.  Since planes
on Intel hardware are tied to a fixed CRTC, go ahead and set state->crtc
to the appropriate crtc in cases where it is passed to us as NULL.

In a future patch, we will start using the update handler for plane
disables, so this will help ensure we always have a non-NULL crtc
pointer to work with.
Signed-off-by: NMatt Roper <matthew.d.roper@intel.com>
Reviewed-by: NAnder Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>

drivers/gpu/drm/i915/intel_display.c

@@ -11898,7 +11898,7 @@ intel_update_plane(struct drm_plane *plane, struct drm_crtc *crtc,
 	struct intel_crtc *intel_crtc = to_intel_crtc(crtc);
 	int ret;
 
-	state.base.crtc = crtc;
+	state.base.crtc = crtc ? crtc : plane->crtc;
 	state.base.fb = fb;
 
 	/* sample coordinates in 16.16 fixed point */