[media] DVB: dvb_frontend: add parameters_out

- Holds the parameters detected by the demod.
- Updated on every call to get_frontend, either through ioctl or when
  a frontend event occurs.
- Reset to input parameters after every call to set_frontend, tune or
  search/track.

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