I found there are 2 solutions:
1. to add 'video=radeonfb:1024x768-16@60 video=vesafb
2. to add 'video=radeonfb
ff video=vesafb vga=791
As I want the bootsplash feature and the first one has some problems with that I will use the second one that does the job well.
Thanks again for your replies!