IB/mlx5: Convert UMR CQ to new CQ API

Simplifies the code, and makes it more fair vs other users by using a
softirq for polling.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Haggai Eran <haggaie@mellanox.com>
Reviewed-by: Sagi Grimberg <sagig@mellanox.com>
Signed-off-by: Doug Ledford <dledford@redhat.com>