[media] uvcvideo: Move mutex lock/unlock inside uvc_free_buffers

Callers outside uvc_queue.c should not be forced to lock/unlock the
queue mutex manually. Move the mutex operations inside
uvc_free_buffers().

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