MetaMonitorManagerKms: stop taking drmModeModeInfo.vrefresh directly

As in the previous commit, this value already has mode flags baked in
and we can always get the value we need by computing it ourselves.

https://bugzilla.gnome.org/show_bug.cgi?id=772176

src/backends/native/meta-monitor-manager-kms.c

@@ -401,9 +401,6 @@ drm_mode_vrefresh (const drmModeModeInfo *mode)
 {
   float refresh = 0.0;
 
-  if (mode->vrefresh > 0.0)
-    return mode->vrefresh;
-
   if (mode->htotal > 0 && mode->vtotal > 0)
     {
       /* Calculate refresh rate in milliHz first for extra precision. */