nouveau: fix storing invalid ptes

nouveau: fix storing invalid ptes

When migrating a range of system memory to device private memory, some of
the pages in the address range may not be migrating. In this case, the non
migrating pages won't have a new GPU MMU entry to store but the
nvif_object_ioctl() NVIF_VMM_V0_PFNMAP method doesn't check the input and
stores a bad valid GPU page table entry.

Fix this by skipping the invalid input PTEs when updating the GPU page
tables.

Link: https://lore.kernel.org/r/20200723223004.9586-2-rcampbell@nvidia.com
Signed-off-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>