RDMA/mlx4: Prepare QP allocation to remove from the driver

Since all mlx4 QP have same storage type, move the QP allocation to be in
one place. This change is preparation to removal of such allocation from
the driver.

Link: https://lore.kernel.org/r/20200926102450.2966017-7-leon@kernel.org
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>