AMDGPU's xf86-video-amdgpu vs. Mode-Setting DDX Performance

To complement the xf86-video-ati vs. xf86-video-modesetting benchmarks from quite a while ago, here are some tests of the 3D performance in seeing any performance differences between the xf86-video-amdgpu DDX for the GCN 1.2 GPUs versus using the generic xf86-video-modesetting driver...