nouveau: 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.

Link: https://lore.kernel.org/r/20200701225352.9649-4-rcampbell@nvidia.com


Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>