[media] omap_vout: Drop overlay format enumeration

Enumerating formats for output overlays doesn't make sense, as the pixel
format is defined by the display API, not the V4L2 API. Drop the
vidioc_enum_fmt_vid_overlay ioctl operation.

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>