This driver is out on 1.0. Portage only has an old snapshot at the time. Would be great to provide an updated ebuild. This driver is used in newest AMD video cards.
1.0.1 was released 4 days ago.
commit 21779ce92c01f5c02bfd76cb30fd2eafe3dcb161 Author: Chí-Thanh Christopher Nguyễn <chithanh@gentoo.org> Date: Tue Feb 23 16:19:34 2016 +0100 x11-drivers/xf86-video-amdgpu: Version bump. Bug: https://bugs.gentoo.org/show_bug.cgi?id=568274 Package-Manager: portage-2.2.26