V4L/DVB: v4l: add new subdev vbi ops to handle raw and sliced vbi

Currently raw and sliced vbi is setup using the video g/s_fmt ops.
However, we want to split off the vbi formatting into separate ops
so the first step is to create those new ops.

The next step will be to convert subdev drivers and bridge drivers
to use the new ops. And in the final step we can start converting
the video g/s_fmt ops to the new (and more appropriate) mediabus
format ops.

Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>