Using displacements is a bit of an overkill imo. If we take a look at the wireframe of a sawmill stair model, all steps consist of just 2 "blocks".
I'll describe a better (IMO) way to make stairs like these.
1. Make stairs with 12x8 units step size. Use 3 units high brushes for steps.
2. Clip the steps in half.
3. Drag the middle vertices of steps 3 units down.
4. Drag the middle vertices of a few steps 1 more unit down.
5. Drag the middle vertices of each step individually random distance left or right.
6. Clip off 1/3 of a few steps.
End result:
