drm/nouveau/kms/nv50: use "low res" lut for indexed mode

This is what was done prior to the big refactor in kernel 3.8, and what
was done by xf86-video-nv. Experimentally, it seems like there's some
sort of issue with the high-res lut to be used there. Tested on G92.

This ends up mattering because fbcon/fbdev chooses the C8 fb format for
low-vram systems, which can occur with some IGP-based configurations
with little "stolen" vram.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=80675
Signed-off-by: Ilia Mirkin <imirkin@alum.mit.edu>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>