That's the sign of your displacement texture lacking bit depth. Displacement textures should be saved to at least 16 bit file formats, preferably to 32 bit formats. If you don't have an option to re-author problematic texture, then you can try to load it in photoshop, convert to 16 bit and then apply slight amount of blur. This operation will slightly degrade your texture, but should help to get rid of those nasty stairs. Not an ideal solution, but better than nothing.
indeed this is the problem thank you very much for your help