[media] media: use media_gobj inside links

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: Hans Verkuil <hans.verkuil@cisco.com>
Tested-by: Javier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>