[media] ivtv: add IVTV_IOC_PASSTHROUGH_MODE

With this private ioctl it is possible to use the ivtv decoder without
requiring the dvb/video.h and dvb/audio.h headers.

Eventually support for those DVB APIs will be dropped from ivtv.

Signed-off-by: Hans Verkuil <hans.verkuil@cisco.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>