media: common: saa7146: drop 'fmt' from struct saa7146_buf

Use the video_fmt in saa7146_vv instead of having a pointer
to it in struct saa7146_buf.

Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>