nvme-pci: move the HMPRE check into nvme_setup_host_mem

Check that a HMB is wanted into the allocation helper instead of the
caller.  This makes life simpler for an upcoming second caller.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Reviewed-by: Chaitanya Kulkarni <kch@nvidia.com>