drm: plane: Clear plane.crtc and plane.fb after disable_plane()

These are the only indication to user space that the plane was disabled.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Dave Airlie <airlied@redhat.com>