nouveau: use alloc_page_vma directly

hmm_vma_alloc_locked_page is scheduled to go away, use the proper
mm function directly.

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