V4L/DVB (11622): cx18: Allow IVTV format VBI insertion in MPEG-2 SVCD and DVD streams

Both the MPEG-2 SVCD stream format and the MPEG-2 DVD stream format should
use an MPEG-2 PS container.  This makes it safe to stuff IVTV Private Stream 1
VBI packets in these stream types using the existing cx18 driver routines.

Reported-by: Helen Buus <mythtv@hbuus.com>
Signed-off-by: Andy Walls <awalls@radix.net>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>