[media] ivtv: disable fbuf support if ivtvfb isn't loaded

Disable all fbuf-related functionality if ivtvfb isn't loaded. This
caused various v4l2-compliance failures.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@osg.samsung.com>