[media] v4l: vsp1: create pad links after subdev registration

The vsp1 driver creates the pads links before the media entities are
registered with the media device. This doesn't work now that object
IDs are used to create links so the media_device has to be set.

Move entities registration logic before pads links creation.

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>