RDMA/mlx5: Update all DRIVER QP places to use QP subtype

Instead of overwriting QP init attributes with driver QP subtype,
use that subtype directly. This change will allow us to remove
logic which cached QP init attributes.

Link: https://lore.kernel.org/r/20200427154636.381474-10-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>