We also have the same question.
We use an old version of LiquidVR to enable direct mode (direct-to-display) for our VR HMD. The person who first received the SDK from AMD is no longer at the company, so we're not sure by what means they got it.
It would be helpful if someone from AMD can link information regarding where HMD makers can apply for the relevant APIs/SDKs for headset makers.