RDMA/mlx5: Separate DCI QP creation logic

RDMA/mlx5: Separate DCI QP creation logic

This patch isolates DCI QP creation logic to separate function, so this
change will reduce complexity when adding new features to DCI QP without
interfering with other QP types.

The code was copied from create_user_qp() while taking only DCI relevant bits.

Link: https://lore.kernel.org/r/b4530bdd999349c59691224f016ff1efb5dc3b92.1624258894.git.leonro@nvidia.com
Reviewed-by: Meir Lichtinger <meirl@nvidia.com>
Signed-off-by: Lior Nahmanson <liorna@nvidia.com>
Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>