drm/i915/gvt: Prevent divided by zero when calculating refresh rate

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.com
Reviewed-by: Zhenyu Wang <zhenyuw@linux.intel.com>