drm/modes: Avoid divide by zero harder in drm_mode_vrefresh()

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

drm/modes: Avoid divide by zero harder in drm_mode_vrefresh()

drm_mode_vrefresh() is trying to avoid divide by zero

by checking whether htotal or vtotal are zero. But we may

still end up with a div-by-zero of vtotal*htotal*...