(In reply to comment #11)
> If I change /sys/class/backlight/radeon_bl0/brightness or
> /sys/class/backlight/acpi_video0 the display stayes dark.
> 
> @Alex
> is this what you wanted to know? If not, please let me know, how I could
> help you to debug this problem!

Without the patch from this bug report applied (e.g., when your display is
working), which, if any, of the blacklight interfaces work?  It seems like
perhaps your laptop claims to have a GPU controlled backlight, but really uses
an external backlight control.  Can you attach a copy of your vbios?  TO get a
copy of your vbios:

(as root)
(use lspci to get the bus id)
cd /sys/bus/pci/devices/<pci bus id>
echo 1 > rom
cat rom > /tmp/vbios.rom
echo 0 > rom