media: bttv: move vbi_skip/vbi_count out of buffer

Instead of storing vbi_skip and vbi_count in each bttv buffer separately,
move them to the main bttv struct as they won't change per buffer.

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