V4L/DVB (13906): cx18: Start IDX streams automatically as an internal associated stream

This change starts the IDX stream along with the MPG stream as an internal
use (only) stream much like the VBI stream can be started as an internal use
stream for inserting sliced VBI packets.

The IDX stream is not started automatically with an MPEG strem if the IDX
stream is disabled (no buffers allocated) or if sliced VBI insertion is being
performed by the cx18 driver.  The cx18 driver doing sliced VBI insertion
makes the offsets in the IDX stream inaccurate for the final MPEG stream
presented to user space.  Since fixing the IDX offsets ourselves is not easy
and we cannot easily do what ivtv does to fix the offsets, we'll make sliced
VBI insertion and MPEG Index capture mutually exclusive for now.

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