mm: simplify ZONE_DEVICE page private data

Remove the clumsy hmm_devmem_page_{get,set}_drvdata helpers, and
instead just access the page directly.  Also make the page data
a void pointer, and thus much easier to use.

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>