V4L/DVB (6755): Avoid troubles when using multiple devices

mv_count is a counter used to move the vertical bars. Before this patch, it
where a static var. This works fine for just one device. However, when using
multiple devices, every device would increment it.

This patch moves it to its correct place: struct vivi_dev. So, now, each device
has its own data.

Signed-off-by: Mauro Carvalho Chehab <mchehab@infradead.org>