To get refresh rate as vblank timer period and keep the precision, the
calculation of rate is multiplied by 1000. However old logic was using:
rate = pixel clock / (h * v / 1000). When the h/v total is invalid, like
all 0, h * v / 1000 will be rounded to 0, which leads to a divided by 0
fault.

0 H/V are already checked above. Instead of divide after divide, refine
the calculation to divide after multiply: "pixel clock * 1000 / (h * v)"
Guest driver should guarantee the correctness of the timing regs' value.

Fixes: 6a4500c7 ("drm/i915/gvt: Get accurate vGPU virtual display refresh rate from vreg")
Reported-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Colin Xu <colin.xu@intel.com>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20210416083355.159305-1-colin.xu@intel.comReviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>