[media] uvcvideo: Implement vb2 queue start and stop stream operations

To work propertly the videobuf2 core code needs to be in charge of
stream start/stop control. Implement the start_streaming and
stop_streaming vb2 operations and move video enable/disable code to
them.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>