@Jim

by using fbdev instead of radeon in xorg.config .

Section "Monitor"
Identifier "Monitor0"
EndSection

Section "Device"
Identifier "Device0"
Driver "fbdev"
EndSection

Section "Screen"
Identifier "Screen0"
Device "Device0"
Monitor "Monitor0"
DefaultDepth 24
SubSection "Display"
Viewport 0 0
Depth 24
EndSubSection
EndSection