nvme-pci: only allow polling with separate poll queues

This will allow us to simplify both the regular NVMe interrupt handler
and the upcoming aio poll code.  In addition to that the separate
queues are generally a good idea for performance reasons.

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