[media] omap_vout: Use the output overlay ioctl operations

The omap_vout device implements the output overlay API, use the
corresponding ioctl operations.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>