[media] V4L-DVB: tm6000: Move VBI init to a separate function

While here, documment that VBI may need changes, based on video STD, and
do some cleanup at device init, to be sure that VBI init will happen
all the times, and to remove a duplicated video standard call.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>