RDMA/mlx5: Use mlx5_umem_find_best_quantized_pgoff() for QP

Delete custom logic in the QP in favor of more general variant.

Link: https://lore.kernel.org/r/20201115114311.136250-5-leon@kernel.org


Signed-off-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>