mm/hmm: make HMM_MIRROR an implicit option

Make HMM_MIRROR an option that is selected by drivers wanting to use it
instead of a user visible option as it is just a low-level implementation
detail.

Link: https://lore.kernel.org/r/20190806160554.14046-15-hch@lst.de
Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>