V4L/DVB (6345): ivtvfb: YUV handling of an image which is not visible in the display area

When the ivtvfb module is loaded, the YUV output is relative to the
framebuffer output. When a virtual screen size is used, the output area for
the YUV may actually be off screen. To prevent the hardware from crashing,
the current driver will ignore an off-screen position and leave the output
visible at the last on-screen position. This may not be desirable, so this
patch will switch off the YUV output should the image move off-screen, and
re-enable it should the image move on-screen again.

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>