drm/i915: Ensure crtc_state backpointer is always initialized

As we transition to full atomic modesetting, we want to be able to pass
intel_crtc_state around in various places that we pass intel_crtc
directly today.  Ensure that the ->crtc backpointer is properly
initialized in case we need to get back to the associated CRTC.

Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>