disabling atomic support for modesetting DDX breaks vsync on Gemini Lake

Since f0d78b47 , vsync is completely broken for me on Gemini Lake with Mesa 19.1.5. It seems to affect most, if not all programs. E.g. mpv with vsync via GL or Compton with vsync via xrender Present extension. mpv in fullscreen triggers pageflipping and thus should also be free of tearing without a compositor or TearFree. Reverting that commit fixes the tearing (though there is stutter in fixed interval in mpv with --video-sync=display-resample , which is not there with xf86-video-intel).

Edit: Modesetting with a Polaris GPU doesn't seem to be affected (both the current tearing issue without atomic support and the regular interval stutter in mpv).