drm/nv50/kms: use dac loadval from vbios, where it's available

Regression from merging the old nv50/nvd9 code together, and may be
needed to fully fix fdo#64904.

The value is ignored completely by the hardware starting from nva3.

Reported-by: Emil Velikov <emil.l.velikov@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>