[media] au0828: continue video streaming even when no ITU-656 coming in

We need the au0828 to continue delivering frames even when the device is not
delivering video, or else applications such as tvtime will block indefinitely.

Unfortunately, the au8522 doesn't have any sort of free-running mode or "blue
screen on no video" like some other decoders.

This work was sponsored by GetWellNetwork Inc.

Signed-off-by: Devin Heitmueller <dheitmueller@kernellabs.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>