drm/nouveau/vmm/gp100-: fix mapping 2MB sysmem pages

The nvif_object_ioctl() method NVIF_VMM_V0_PFNMAP wasn't correctly
setting the hardware specific GPU page table entries for 2MB sized
pages. Fix this by adding functions to set and clear PD0 GPU page
table entries.

Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>