Add new fields to struct media_device to add enable_source, and
disable_source handlers, and source_priv to stash driver private
data that is used to run these handlers. The enable_source handler
finds source entity for the passed in entity and checks if it is
available. When link is found, it activates it. Disable source
handler deactivates the link.

Bridge driver is expected to implement and set these handlers.
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>