backends/drm: add workaround for an amdgpu adaptive sync cursor bug

See https://gitlab.freedesktop.org/drm/amd/-/issues/3034 for details. I'll try to fix it upstream once more, but if there isn't a patch at least on its way to be merged in 2-3 weeks, I'll merge this so that Plasma 6 doesn't have broken always-on vrr.

cc #194