V4L/DVB (13907): cx18: Perform automatic rotation of very old, unread IDX buffers

According to the v4l2 spec, very old MPEG index entries needs to be discarded
in favor of newer index entries.  This change ensures the firmware always has
buffers for index entries at the expense of the oldest unread buffers.

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