fbdev: tdfxfb: avoid divide-by-zero on FBIOPUT_VSCREENINFO

In the Linux kernel, the following vulnerability has been resolved:

fbdev: tdfxfb: avoid divide-by-zero on FBIOPUT_VSCREENINFO

Much like commit 19f953e74356 ("fbdev: fb_pm2fb: Avoid potential divide

by zero error"), we also need to prevent that same crash from happening

in the udlfb driver as it uses pixclock directly when dividing, which

will crash.