Fix divide by zero error for nvidia 7600 pci-express card

The following patch resolves the divide by zero error I encountered on my
system:

        http://marc.10east.com/?l=linux-fbdev-devel&m=116058257024413&w=2

I accomplished this by merging what I thought was appropriate from:

        http://webcvs.freedesktop.org/xorg/driver/xf86-video-nv/src/



Signed-off-by: Adrian Bunk <bunk@stusta.de>