Those callbacks are called with the media_device.graph_mutex held.

Add a note about that, as the code called by those notifiers should
not be touching in the mutex.
Acked-by: NSakari Ailus <sakari.ailus@linux.intel.com>
Acked-by: NHans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: NMauro Carvalho Chehab <mchehab@osg.samsung.com>