According to http://www.x.org/wiki/Development/Security/Advisory-2014-12-09/ issues won't be fixed in new xorg-server-1.15 releases (only 1.16 and later). The today released ati-drivers adds support for Xorg/Xserver 7.4 and above (up to 1.16) plus Linux kernel 2.6 or above (up to 3.17). Can you please provide a version bump?
Bumped in x11 overlay, xorg-server-1.16 (and linux-3.18) seems to work also.
*ati-drivers-14.12 (10 Dec 2014) 10 Dec 2014; Manuel Rüger <mrueg@gentoo.org> +ati-drivers-14.12.ebuild: Proxy commit for Emil Karlson. Version bump. Fixes bug #532068.