drm/i915: Kill intel_plane->obj

intel_plane->obj is not used anymore so kill it. Also don't pass both
the fb and obj to the sprite .update_plane() hook, as just passing the fb
is enough.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>