I think, I will try this:
"For nVidia cards use the yaboot parameter nouveau.modeset=0 to disable KMS. This will disable both the nouveau xorg driver and the nouveaufb framebuffer (as KMS and nouveaufb are also inseparable) and you will fallback to the fbdev/offb driver. Note the openfirmware framebuffer has a limited colour depth - hence the ‘psychedelic’ colours you will see. You can increase the colour depth with fbdev by using a different framebuffer (such as nouveaufb or the legacy framebuffers nvidiafb or rivafb) and creating an xorg.conf that specifies fbdev, but if you can’t get KMS fully working then the better solution is to use the nv driver in your xorg.conf. "