The problem is that /etc/modprobe.d/radeon-kms.conf explicitly specifies radeon
modeset=1 so maybe we should add radeon.modeset=0 to the boot parameters.

For debugging this issue
http://wiki.x.org/wiki/radeonBuildHowTo#Moduleforframebufferconsole helps.

Stuff we maybe can try:
radeon.agpmode=-1
remove the vga parameter specify radeon.modeset=0, boot, unload the all radeon
and fb modules, load the radeon module again.