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: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Ander Conselvan de Oliveira <ander.conselvan.de.oliveira@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>