[media] omap_vout: fix crash if no driver for a display

omap_vout crashes on start if a corresponding driver is not loaded for a
display device.

This patch changes omap_vout init sequence to skip devices without a
driver.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>