[media] DVB: call get_property at the end of dtv_property_process_get

- Drivers should be able to override properties returned to the user.
- The default values get prefilled from the cache.

Signed-off-by: Andreas Oberritter <obi@linuxtv.org>
Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>