present: Fix missed notify MSC computation

Only treat divisor==0 as async to immediately report the actual vblank.
If the user species a non-zero divisor, we should compute the missed
vblank properly or else we report too early.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mario Kleiner <mario.kleiner.de@gmail.com>

present/present.c

@@ -933,7 +933,7 @@ present_notify_msc(WindowPtr window,
                           0, 0,
                           NULL,
                           NULL, NULL,
-                          PresentOptionAsync,
+                          divisor == 0 ? PresentOptionAsync : 0,
                           target_msc, divisor, remainder, NULL, 0);
 }