[media] media: Don't accept early-created links

Links are graph objects that represent the links of two already
existing objects in the graph.

While with the current implementation, it is possible to create
the links earlier, It doesn't make any sense to allow linking
two objects when they are not both created.

So, remove the code that would be handling those early-created
links and add a BUG_ON() to ensure that.

Acked-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>