RDMA/mlx5: Prepare QP allocation for future removal

Unify the QP memory allocation across different paths,
so it will be in one place.

Link: https://lore.kernel.org/r/20200427154636.381474-5-leon@kernel.org
Reviewed-by: Maor Gottlieb <maorg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>