It's on by default (this means existing scenes will have better quality displacement - you can also lower the displacement quality setting for the scene (ie go from 2px to 4px say) and get comparable quality for your existing scene for significantly less memory usage.
If you want to disable it (though, no reason to, to be honest), it is in the Devel/Debug options, since it is not one that should need adjusting by the user :)
EDIT - Haha that will teach me to reply from random airports while traveling :) The latest daily build yesterday disables Autobump for existing scenes when loaded, with a dialog to enable you to turn it on if you want (see
https://forum.corona-renderer.com/index.php?topic=20852.msg135716#msg135716)
