Just like entities and pads, links also need to have unique
Object IDs along a given media controller.

So, let's add a media_gobj inside it and initialize
the object then a new link is created.
Acked-by: NHans Verkuil <hans.verkuil@cisco.com>
Tested-by: NJavier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>