fbdev: omapfb: use of_graph_get_remote_port()

We already have of_graph_get_remote_port(), Let's use it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Helge Deller <deller@gmx.de>