RDMA/mlx5: Use direct modify QP implementation

As a preparation to removal hand crafted mlx5_qp_context, convert counter
code to use mlx5_cmd_exec_in() directly.

Link: https://lore.kernel.org/r/20200526115440.205922-5-leon@kernel.org
Reviewed-by: Mark Zhang <markz@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>