RDMA/mlx5: Split scatter CQE configuration for DCT QP

DCT QPs have separate creation flow and can be easily extracted
from configure_responder_scat_cqe(), this makes both updated
functions more clear.

Link: https://lore.kernel.org/r/20200427154636.381474-9-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>