drm/nouveau/disp/dp: make use of postcursor when its available
drm/nouveau/disp/dp: make use of postcursor when its available

And at the same time, obey the spec better wrt out-of-range requests.

Signed-off-by: Ben Skeggs <d9f27fb07c1e9f131223ad827fa5179f3846c30b@redhat.com>

KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs