[media] dib8000: store dtv_property_cache in a temp var

dtv_property_cache is used on several places on very long lines.

On all places it is used, a long list of struct reference is done.

Instead of doing it, at the routines where it is used more than once,
replace it by one temporary var. That may help the compiler to
use a better code. It also makes easier to review the code, as the
lines becomes closer to 80 columns, making them a way clearer
to read.

No functional changes.

Signed-off-by: Mauro Carvalho Chehab <mchehab@redhat.com>