drm/nouveau/disp: take sink support into account for exposing 594mhz

Scrambling is required for supporting any mode over 340MHz. If it's not
supported, reject any modes that would require it.

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