modesetting: unflip not possible when glamor is not set

This is fixing crashes of xfce when running under qemu

hw/xfree86/drivers/modesetting/present.c

@@ -272,6 +272,9 @@ ms_present_check_unflip(RRCrtcPtr crtc,
         pixmap->devKind != drmmode_bo_get_pitch(&ms->drmmode.front_bo))
         return FALSE;
 
+    if (!ms->drmmode.glamor)
+        return FALSE;
+
 #ifdef GBM_BO_WITH_MODIFIERS
     /* Check if buffer format/modifier is supported by all active CRTCs */
     gbm = ms->glamor.gbm_bo_from_pixmap(screen, pixmap);