mm: don't select MIGRATE_VMA_HELPER from HMM_MIRROR

The migrate_vma helper is only used by noveau to migrate device private
pages around.  Other HMM_MIRROR users like amdgpu or infiniband don't
need it.

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>