[media] vivid: move video loopback control to the capture device

This has been on my TODO list for some time now: the control that enables the video
loopback was part of the controls of the video output device instead of the video
capture device. In practice this was quite annoying since you expect it at the capture
side since that's where you want to make the decision whether to use the TPG or looped
video.

This patch moves the control from the output to the capture side.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>