[media] media-device: move media entity register/unregister functions

media entity register and unregister functions are called by media
device register/unregister. Move them to occur earlier, as we'll need
an unlocked version of media_device_entity_unregister() and we don't
want to add a function prototype without needing it.

No functional changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>