As stated here: http://www.phoronix.com/scan.php?page=news_item&px=NTkyMg
you will have to:
- Download the latest ati driver for your graphics card
- Extract the driver by executing the downloaded *.run file with option --extract in your terminal
- Go to the extracted folder and find common/etc/ati/control
- Copy the file named control into /etc/ati on your file system
With the latest control file the watermark should be gone.
This solved the problem for me.