sna/video/sprite: Remove stale video->plane

video->plane is never populated so nuke it. Its only user can be nuked
as well since all it's trying to do is turn off the plane (which fails
on account plane_id being zero). We don't need to disable the plane
immediately upon the setplane failure as the higher level code will
end up stopping the entire port on error, and thus the plane will get
disabled just fine.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>