drm: zte: Remove unnecessary drm_plane_cleanup() wrapper

Use the drm_plane_cleanup() function directly as the drm_plane_funcs
.destroy() handler without creating an unnecessary wrapper around it.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>