Jeff Smith who originally posted the HDMI Stereo 3D patches last month has gone through and revised his code. Additionally, he's wired up this HDMI feature for both DC and non-DC code-paths. For both display code paths, getting HDMI Stereo 3D working is just a little over one hundred lines of code in total.
HDMI 1.4 introduced stereoscopic 3D formats and besides needing a supported AMDGPU-powered graphics card you'll also need to have a 3D TV. Intel's DRM driver has offered Stereo 3D support for a while and there are also Nouveau HDMI 3D patches but not many Linux users (or anyone in general) seem to have 3D TVs.
These latest AMDGPU Stereo 3D patches can be found on the dri-devel list.