If you can find where they host their mesa fork, you can see what they've added there to make it work.
A Google search yielded the following two URLs, but I get "Access denied":
A forum is opened, because Valve talked with Amd.
Amd promised Valve they would listen.
AMD has opened a new forum (http://devgurus.amd.com/community/steam-linux) for those of you using Linux AMD graphics drivers. The purpose of the AMD forum is to provide a direct line to AMD for feedback on their graphics drivers.
Currently before you can post you will need to create a new AMD forum account and request access to the forum. Currently, the forum is private but that will change to public very soon.
In case you are wondering, we will continue monitoring this forum for AMD-related issues with Steam for Linux.