sna: Drain all udev events before checking output status

As libudev only pulls off events one by one and we may need to process
several for a MST topology change, keep polling until complete.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

src/sna/sna_driver.c

@@ -879,7 +879,7 @@ static bool sna_uevent_poll(struct sna *sna)
 	pfd.fd = udev_monitor_get_fd(sna->uevent_monitor);
 	pfd.events = POLLIN;
 
-	if (poll(&pfd, 1, 0) > 0)
+	while (poll(&pfd, 1, 0) > 0)
 		sna_handle_uevents(pfd.fd, sna);
 
 	return true;