drm: rcar-du: Use the drm atomic state duplication helpers for planes

Ensure that the duplicate and destroy plane state operations will always
be in sync with the DRM core implementation of the plane state by using
the __drm_atomic_helper_plane_duplicate_state() and
__drm_atomic_helper_plane_destroy_state() functions designed especially
for this purpose.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>