OMAPDSS: RFBI: Mark RFBI as broken

OMAPDSS's RFBI encoder driver has not been working for some time. The
Nokia N800 is the only board in the mainline that uses RFBI, but it has
never been fully functional.

The RFBI driver needs to be updated to the new panel model, but as
the driver is rather unmaintained and there's no way to test the
changes, let's mark the driver as broken.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Archit Taneja <archit@ti.com>