https://bugs.freedesktop.org/show_bug.cgi?id=80531
--- Comment #12 from Alex Deucher agd5f@yahoo.com --- Created attachment 102075 --> https://bugs.freedesktop.org/attachment.cgi?id=102075&action=edit add deep color module option
This patch disables the deep color support by default which should resolve your issue. You can set radeon.deep_color=1 on the kernel command line in grub to re-enable it. Hopefully we can resolve this properly for 3.17 and enable deep color by default.