drm/i915: Deal with video overlay on GPU reset

Clear the video overlay state on GPU reset. Any pending overlay request
in the ring has been nuked, and the display itself gets reset. So we
pretty much lose all state here. Adjust the software state to match so
that the next "putimage" will restore things to working order.

v2: Ass a locking check into intel_overlay_release_old_vid() (Daniel)

Cc: Daniel Vetter <daniel@ffwll.ch>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
[danvet: s/0/NULL/ to appease sparse, reported by 0-day tester.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>