At least AMD claims that it has working support for newer xservers. "Features Xserver 1.14 Support" See $URL
Created attachment 349714 [details, diff] Patch to remove dependency on x11 1.13
I've applied the patch to remove this dependency and can confirm that the driver works successfully on xorg-server-1.14 on an amd64 system.
Emil has fixed it already in the overlay. It just needs to be moved to the main tree. http://git.overlays.gentoo.org/gitweb/?p=proj/x11.git;a=commit;h=b8bf5ee1760c329ad7a8bc4d336e112a64b712bc
Fixed in CVS.