mm/hmm: remove the legacy hmm_pfn_* APIs

Switch the one remaining user in nouveau over to its replacement, and
remove all the wrappers.

Link: https://lore.kernel.org/r/20190724065258.16603-7-hch@lst.de


Tested-by: Ralph Campbell <rcampbell@nvidia.com>
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Ralph Campbell <rcampbell@nvidia.com>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>