net/mlx5: Replace hand written QP context struct with automatic getters

By changing debugfs to not use any QP related API, convert the debugfs
code to use MLX5_GET() directly to access QP context instead of hand
written struct.

Reviewed-by: Saeed Mahameed <saeedm@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>