I don't believe the curse was any curse at all. Going by the description of Draigo, M'kar just grabbed him and threw him into the warp. I do not believe this is a direct result of M'kar's curse; would the Chaos Gods really have gone through with it if they had known how much trouble Draigo would cause in their realms? If he did in fact curse Draigo, he basically shot Chaos in the foot.
It is entirely possible that it was the Daemon Prince M'Kar back-up plan all along. You see Draigo bust the faces of all the other gods, including a possible rival in the Lord of Change M'Kachen (and his city). It could be a bit of manipulating by M'Kar in an attempt to elevate his own position by weakening others.
Or the Dark Gods have a greater scheme taking place in the Mortal realm and do not want Draigo to interfere with it. By imprisoning him in the Warp you prevent him for leading the Grey Knights versus a grand scheme of destruction against the Imperium.