Add enable_source and disable_source handlers. The enable source handler
is called from v4l2-core, dvb-core, and ALSA drivers to check if the
shared media source is free.

The disable source handler is called to release the shared media
source.

[mchehab@osg.samsung.com: fix merge conflicts]
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>