backends/drm: work around amdgpu vrr cursor bug differently
Instead of not-delaying cursor updates with adaptive sync, this forces a software cursor instead. That way, the functionality works the same on all the vendors.
For testing potential driver fixes, the environment variable KWIN_DRM_DONT_FORCE_AMD_SW_CURSOR=1 can be used to disable this workaround
cc #194 closes #85 (closed)