V4L/DVB (9326): ivtv: avoid green flashing when loading ivtv

When loading ivtv the TV-out of the PVR-350 will flash green since the
saa712x is activated before the MPEG decoder has been initialized.

Deactivate the saa712x until the MPEG decoder has been initialized.

Signed-off-by: Martin Dauskardt <martin.dauskardt@gmx.de>
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>