OMAP: OMAPFB: make omapfb start even when a display is missing a driver

Currently omapfb wants that all the display devices have a driver,
otherwise omapfb refuses to start. There's no real requirement to act
like that, and this patch will make omapfb give a warning and skip that
device.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>