nouveau/hmm: support mapping large sysmem pages

Nouveau currently only supports mapping PAGE_SIZE sized pages of system
memory when shared virtual memory (SVM) is enabled. Use the new
hmm_pfn_to_map_order() function to support mapping system memory pages
that are PMD_SIZE.

Link: https://lore.kernel.org/r/20200701225352.9649-5-rcampbell@nvidia.com


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