media: bttv: copy vbi_fmt from bttv_fh

In preparation for the vb2 conversion, copy the vbi format from struct
bttv_fh and add it to the main struct bttv. Use vbi format from struct
bttv wherever it will be needed after the vb2 conversion which stops using
separate bttv file handles altogether. To avoid changing more code than
necessary, just leave the vbi format in separate file handles wherever it
will be subsequently removed by vb2.

Signed-off-by: Deborah Brouwer <deborah.brouwer@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>