drm/i915/dp: Explicitly request 8/10 channel coding

It's not clear what a sink would do if you wrote zero to this register -
which I guess would mean "I don't support any channel encodings, good
luck" - but let's not find out.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Reviewed-by: Keith Packard <keithp@keithp.com>
Signed-off-by: Keith Packard <keithp@keithp.com>