drm/omap: remove rfbi

The RFBI driver has not worked nor compiled for many years. There are
very few boards out there that use RFBI, and no one has stepped up to
fix it.

So let's remove the RFBI code that doesn't even compile.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>