V4L/DVB (13905): cx18: Allow MPEG index streams to be started and stopped internally

This change allows the IDX stream to be started and stopped as any other
stream even though it has no associated device node.  This is needed for
cx18 driver internal use.

Also always tell the CX23418 to generate index entries when an analog capture
starts and the IDX stream has had buffers allocated (i.e. is enabled).

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