wayland/buffer: Avoid spamming the log when a buffer is not scanoutable

It's not really an error and we recover seamlessly.

If someone really wants to check if/why direct scanout is failing then
they can still use `env MUTTER_DEBUG=render,kms`.

Fixes: https://gitlab.gnome.org/GNOME/mutter/-/issues/2702
Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2918>

src/wayland/meta-wayland-buffer.c

@@ -877,10 +877,9 @@ meta_wayland_buffer_try_acquire_scanout (MetaWaylandBuffer *buffer,
       return NULL;
     }
 
-  g_return_val_if_fail (scanout, NULL);
-
-  g_signal_connect (scanout, "scanout-failed",
-                    G_CALLBACK (on_scanout_failed), buffer);
+  if (scanout)
+    g_signal_connect (scanout, "scanout-failed",
+                      G_CALLBACK (on_scanout_failed), buffer);
 
   return scanout;
 }