drm: Warn about negative sizes when calculating scale factor

Passing negative width/hight to scale factor calculations is not
legal. Let's WARN if that happens.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NSean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1469549224-1860-2-git-send-email-ville.syrjala@linux.intel.com

drivers/gpu/drm/drm_rect.c

@@ -100,7 +100,7 @@ static int drm_calc_scale(int src, int dst)
 {
 	int scale = 0;
 
-	if (src < 0 || dst < 0)
+	if (WARN_ON(src < 0 || dst < 0))
 		return -EINVAL;
 
 	if (dst == 0)