[media] OMAP_VOUT: Change hardcoded device node number to -1

With addition of media-controller framework, now we have various
device nodes (/dev/videoX) getting created, so hardcoding
minor number in video_register_device() is not recommended.

So let V4L2 framework choose free minor number for the device.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>