nouveau: use devm_memremap_pages directly

Just use devm_memremap_pages instead of hmm_devmem_add pages to allow
killing that wrapper which doesn't provide a whole lot of benefits.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>