drm/nouveau/disp/dp: support postcursor in link training

Not enabled at the backends yet, but will read status and send back max
reached at level 0.

Signed-off-by: Ben Skeggs <bskeggs@redhat.com>

diff --cc drivers/gpu/drm/nouveau/core/engine/disp/dport.c
Simple merge

diff --cc drivers/gpu/drm/nouveau/core/engine/disp/dport.h
Simple merge