drm/nouveau/pm/gf100: remove undefined TEX.PM_UNKC8 mux

This mux only exists on GF108+ (except for GF110 one), but since it is
not used by the userspace we can drop it for now.

Signed-off-by: Samuel Pitoiset <samuel.pitoiset@gmail.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>