[media] media: use media_gobj inside pads

PADs also need unique object IDs that won't conflict with
the entity object IDs.

The pad objects are currently created via media_entity_init()
and, once created, never change.

While this will likely change in the future in order to
support dynamic changes, for now we'll keep PADs as arrays
and initialize the media_gobj embedded structs when
registering the entity.

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>