omapfb: 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>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Signed-off-by: Bartlomiej Zolnierkiewicz <b.zolnierkie@samsung.com>