Comment # 11 on bug 981358 from Robby Engelmann

my /etc/X11/xorg.conf.d/50-device.conf looks like this, which means nothing
defined?

# Having multiple "Device" sections is known to be problematic. Make
# sure you don't have in use another one laying around e.g. in another
# xorg.conf.d file or even a generic xorg.conf file. More details can
# be found in https://bugs.freedesktop.org/show_bug.cgi?id=32430.
#
#Section "Device"
#  Identifier "Default Device"
#
#  #Driver "radeon"
#
#  ## Required magic for radeon/radeonhd drivers; output name
#  ## (here: "DVI-0") can be figured out via 'xrandr -q'
#  #Option "monitor-DVI-0" "Default Monitor"
#
#EndSection