With the "scaling mode" drm property set to "Full Aspect", modesets don't correctly change the refresh rate

KWin generates a list of common modes with libxcvt, which not all displays will handle correctly. To ensure that they do all work, starting with 5.27.4 I made KWin set the "scaling mode" property to "Full Aspect", so that scaling is done by the GPU and not by the display.

This caused problems for some of our users (https://bugs.kde.org/show_bug.cgi?id=468235), namely that after the initial modeset, the displays won't go down or up in the refresh rate anymore. Changing the scaling mode back to "None" and leaving the rest of the changes to KWin intact works around the problem, so it's certain that this is the only factor at play.

Note that while the display won't change the refresh rate, the display blanks for a moment and the kernel tells userspace that the modeset was successful.

Edit: it also happens without FreeSync