sna: Don't attempt to flush scanout if unattached

This was lost in the midst of the OSTimer overhaul.

Reported-by: Zdenek Kabelac <zdenek.kabelac@gmail.com>
References: https://bugs.freedesktop.org/show_bug.cgi?id=50393
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna_accel.c

@@ -11899,7 +11899,7 @@ static bool sna_accel_do_flush(struct sna *sna)
 	struct sna_pixmap *priv;
 
 	priv = sna_accel_scanout(sna);
-	if (priv == NULL) {
+	if (priv == NULL || priv->gpu_bo == NULL) {
 		DBG(("%s -- no scanout attached\n", __FUNCTION__));
 		sna_accel_disarm_timer(sna, FLUSH_TIMER);
 		return false;