gpu: drm: omapdrm: use new of_graph functions

Now we can use new port related functions for port parsing. Use it.

Signed-off-by: Kuninori Morimoto <kuninori.morimoto.gx@renesas.com>
Reviewed-by: Tomi Valkeinen <tomi.valkeinen+renesas@ideasonboard.com>
Acked-by: Helge Deller <deller@gmx.de>
Link: https://lore.kernel.org/r/87cyjqb5sh.wl-kuninori.morimoto.gx@renesas.com


Signed-off-by: Rob Herring (Arm) <robh@kernel.org>