nvme-pci: disable hmb on idle suspend

An idle suspend may or may not disable host memory access from devices
placed in low power mode. Either way, it should always be safe to
disable the host memory buffer prior to entering the low power mode, and
this should also always be faster than a full device shutdown.

Signed-off-by: Keith Busch <kbusch@kernel.org>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>