V4L/DVB (9328): ivtvfb: FB_BLANK_POWERDOWN turns off video output

When using FBIOBLANK, FB_BLANK_POWERDOWN will now switch off the video output.
Since some televisions turn themselves off after a while with no signal, this
is the closest we can get to power-saving.

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@redhat.com>