drm/nouveau/disp: add a way to configure scrambling/tmds for hdmi 2.0

High pixel clocks are required to use a 40 TMDS divider instead of 10,
and even low ones may optionally use scrambling depending on device
support.

Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>