drm: rcar-du: Use of_graph_get_remote_endpoint()

Now, we can use of_graph_get_remote_endpoint(). Let's use it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>