drm/qxl: fix framebuffer dirty rectangle tracking.

Commit "c0fe07a drm/qxl: rewrite framebuffer support" has a bug in the
dirty rectangle tracking:  Instead of ignoring an empty dirty rectangle
when adding a new dirty region the dirty region gets extended to the
upper left corner.  Fix it.

Cc: linux-stable@vger.kernel.org
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>

drivers/gpu/drm/qxl/qxl_fb.c

@@ -144,14 +144,17 @@ static void qxl_dirty_update(struct qxl_fbdev *qfbdev,
 
 	spin_lock_irqsave(&qfbdev->dirty.lock, flags);
 
-	if (qfbdev->dirty.y1 < y)
-		y = qfbdev->dirty.y1;
-	if (qfbdev->dirty.y2 > y2)
-		y2 = qfbdev->dirty.y2;
-	if (qfbdev->dirty.x1 < x)
-		x = qfbdev->dirty.x1;
-	if (qfbdev->dirty.x2 > x2)
-		x2 = qfbdev->dirty.x2;
+	if ((qfbdev->dirty.y2 - qfbdev->dirty.y1) &&
+	    (qfbdev->dirty.x2 - qfbdev->dirty.x1)) {
+		if (qfbdev->dirty.y1 < y)
+			y = qfbdev->dirty.y1;
+		if (qfbdev->dirty.y2 > y2)
+			y2 = qfbdev->dirty.y2;
+		if (qfbdev->dirty.x1 < x)
+			x = qfbdev->dirty.x1;
+		if (qfbdev->dirty.x2 > x2)
+			x2 = qfbdev->dirty.x2;
+	}
 
 	qfbdev->dirty.x1 = x;
 	qfbdev->dirty.x2 = x2;