https://bugs.freedesktop.org/show_bug.cgi?id=40576
--- Comment #1 from Michel Dänzer michel@daenzer.net 2011-09-02 06:06:47 PDT --- (In reply to comment #1)
CONFIG_DRM_RADEON=y
In that case KMS requires the microcode from /lib/firmware/radeon to be built into the kernel as well. Are you doing that?