drm/nouveau/kms/nv50-: add fp16 scanout support

Older hardware seems to want 0..1024 values, while new hardware takes
0..1 values. We set the gain to 1024 for the earlier display classes.

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