[media] uvcvideo: Separate video and queue enable/disable operations

In order to make use of the vb2 queue start/stop_streaming operations
the video and queue enable/disable operations need to be split, as the
vb2 queue will need to enable and disable video instead of the other way
around.

Also move buffer queue disable outside of uvc_video_resume() to remove
all queue disable operations out of uvc_video.c.

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