[media] dvbdev: move indirect links on dvr/demux to a separate function

Cleanup the code a little bit by moving the routine that creates
links between DVR and demux to the I/O entitis into a separate
function.

While here, fix the code to use strncmp() instead of strcmp().

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