drm/nouveau/kms/nv50: use INTERPOLATE_257_UNITY_RANGE LUT on newer chipsets

For some reason forgotten by history, we've been using a 1025-entry LUT
mode, and sparsely filling it with the 256-entry LUT we're handed.

Until we land support for the full atomic colour management properties,
this commit switches to using the 257-entry mode to fix colour mapping
with depth 30 framebuffers.

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