kms-impl/simple: Fix page_flip_data ref leak on fallback

If drmModePageFlip() or custom_page_flip_func fails, process_page_flip() was
forgetting to undo the ref taken for that call. This would leak page_flip_data.

The reference counting works like this:
- when created, ref count is 1
- when calling drmModePageFlip, ref count is increased to 2
- new: if flip failed, ref count is decreased back to 1
- if calling schedule_retry_page_flip(), it takes a ref internally
- if calling mode_set_fallback(), it takes a ref internally
- all return FALSE paths have an explicit unref
- return TRUE path has an explicit unref

This issue was found by code inspection and while debugging an unrelated issue
with debug prints sprinkled around. I am not aware of any end-user visible
issues being fixed by this, as the leak is small and probably very rare.

https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/1209

src/backends/native/meta-kms-impl-simple.c

@@ -1,6 +1,6 @@
 /*
  * Copyright (C) 2018-2019 Red Hat
- * Copyright (C) 2019 DisplayLink (UK) Ltd.
+ * Copyright (C) 2019-2020 DisplayLink (UK) Ltd.
  *
  * This program is free software; you can redistribute it and/or
  * modify it under the terms of the GNU General Public License as
@@ -660,6 +660,9 @@ process_page_flip (MetaKmsImpl    *impl,
                              meta_kms_page_flip_data_ref (page_flip_data));
     }
 
+  if (ret != 0)
+    meta_kms_page_flip_data_unref (page_flip_data);
+
   if (ret == -EBUSY)
     {
       CachedModeSet *cached_mode_set;