V4L/DVB (5972): ivtv: prevent yuv register updates from being missed

The yuv output code always compares the new frame position & size with those
of the previous frame. If they are different, a flag is set to request the
yuv output registers be updated when the new frame is displayed.

If the incoming frames are delivered too fast, exhausting the buffers, the
most recent frame already buffered will be discarded. Unfortunately, any
update request will also be discarded. If the new frame matches the size &
position of the now discarded frame, the yuv registers are not flagged for
update & will remain in their old state.

This patch preserves the register update flag in the event that a frame is
dropped.

Signed-off-by: Ian Armstrong <ian@iarmst.demon.co.uk>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>