The X log is probably misleading. cirrus UMS driver is being tried first (although I'm not sure why it still exists, it has been dropped with sle15), but fails since KMS is active (no issue at all), then also fbdev is being tried, but gdm now starts the Xserver for the session as regular user, which cannot access /dev/fb0 (permissions are 660 for root.video). Then modesetting. Here the logfile got truncated! My guess is that it is successful.