RDMA/mlx5: Change GSI QP to have same creation flow like other QPs

There is no reason to have separate create flow for the GSI QP, while
general create_qp routine has all needed checks and ability to allocate
and free the proper struct mlx5_ib_qp.

Link: https://lore.kernel.org/r/20200926102450.2966017-4-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>