OMAPDSS: fix probing if rfbi device is enabled

After the commit 736e60dd

 ("OMAPDSS:
componentize omapdss") the dss core device will wait until all the
subdevices have been successfully probed. However, we don't have a
working driver for RFBI, so if RFBI device exists, omapdss will never
get probed.

All the .dtsi files set RFBI as disabled, except am4372.dtsi. This
causes omapdss probe to not finish on AM4 devices.

This patch makes omapdss driver skip adding rfbi device as a
subcomponent, solving the issue.

This should be reverted when we have a working RFBI driver.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reported-by: Felipe Balbi <balbi@ti.com>