nvme-pci: remove q_dmadev from nvme_queue

We don't need to save the dma device as it's not used in the hot path
and hasn't in a long time. Shrink the struct nvme_queue removing this
unnecessary member.

Signed-off-by: Keith Busch <keith.busch@intel.com>
Reviewed-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Christoph Hellwig <hch@lst.de>