drm/nouveau/mmu/gp10b: use correct implementation

Reported-by: Mikko Perttunen <cyndis@kapsi.fi>
Fixes: 6359c982 ("drm/nouveau/mmu/gp10b: fork from gf100")
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Tested-by: Thierry Reding <treding@nvidia.com>