nvme-pci: factor out the cqe reading mechanics from __nvme_process_cq

Also, maintain a consumed counter to rely on for doorbell and
cqe_seen update instead of directly relying on the cq head and phase.

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