[media] v4l: vsp1: Don't register media device when userspace API is disabled

The media device doesn't need to be exposed to userspace when the VSP is
fully controlled by the DU driver. Don't register it in that case.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@s-opensource.com>