If they have a working driver since 2 years ago, couldn't they just release it to the community? I imagine most gamers would typically be capable/ok with that.
Are they rejecting the driver because of it being open source? There are specific modules I use in my AMD card that require closed proprietary driver add-ons for example such as AMF.
Not defending the HDMI forum here, but perhaps Valve / AMD have a way of including a proprietary blob in SteamOS (I don't think most gamers would care)