From what I’ve gathered (and I take it with a grain of salt) the AMD code relies on some proprietary code by the HDMI standards people that are under strict NDA. I don’t think the implementation is entirely open source, and I don’t think any existing implementation is, either.

In this case, Nvidia’s approach of open sourcing the driver but doing most of the work in signed firmware blobs running on the card may be the best solution for now. I don’t know how many distros are shipping Nvidia’s open source driver, though.