(In reply to comment #11)
> I've got the same notebook as Daniel Kirsten, 

They sold different Amilo Xi 2550, there are even some Amilo Xi 2550 with 
an Nvidia graphic card.

> If I change /sys/class/backlight/radeon_bl0/brightness or
> /sys/class/backlight/acpi_video0 the display stayes dark.

I can control brightness by writing 0-7 into 
/sys/class/backlight/acpi_video0/brightness

You should comment out lines 233-234 in
drivers/gpu/drm/radeon/atombios_encoders.c,
i.e., 

/*  if (bd->props.brightness == 0)
    234                 bd->props.brightness = RADEON_MAX_BL_LEVEL;    */

then it works.

Daniel