[media] smiapp: create pad links after subdev registration

The smiapp driver creates the pads links before the media entity is
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 entity 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>