drm/nouveau/kms/nv50: fix handling of gamma since atomic conversion

We've still been directly using the legacy crtc gamma_set() hook even
after conversion to atomic modesetting.

For userspace clients this was fine, however, fbcon will use the atomic
property when it's running on an atomic driver, which means we miss its
colormap updates - which is particularly bad for 8bpp framebuffers!

This commit converts the driver to use the atomic property + the helper
function implementing the legacy hook on top of atomic.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80675


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