V4L/DVB (8921): ivtv: fix incorrect capability and assorted sliced vbi and video out fmt fixes

- V4L2_CAP_VBI_OUTPUT is not supported by ivtv, remove it.
- ivtv_try_fmt_vid_out also needed to constrain the width/height of MPEG decoder
  window sizes.
- allow empty sliced services.

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