V4L/DVB (7948): videodev: add missing vidioc_try_fmt_sliced_vbi_output and VIDIOC_ENUMOUTPUT handling

There was no vidioc_try_fmt_sliced_vbi_output, instead vidioc_try_fmt_vbi_output
was reused.

The VIDIOC_ENUMOUTPUT handling was missing altogether, even though the callback
existed.
Signed-off-by: Hans Verkuil <hverkuil@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>