gpu-kms: assert on invalid flip attempts

drmModePageFlip() is guaranteed to fail for the invalid FB id 0.
Therefore it never makes sense to call this function with such argument.
Disabling a CRTC must be done with SetCrtc instead, for example.

Trying to flip to FB 0 not only fails, but it also causes Mutter to
never try page flip on this output again, using drmModeSetCrtc()
instead.

src/backends/native/meta-gpu-kms.c

@@ -2,6 +2,7 @@
 
 /*
  * Copyright (C) 2017 Red Hat
+ * Copyright (c) 2018 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
@@ -257,6 +258,8 @@ meta_gpu_kms_flip_crtc (MetaGpuKms *gpu_kms,
   g_assert (n_connectors > 0);
   g_free (connectors);
 
+  g_assert (fb_id != 0);
+
   if (!gpu_kms->page_flips_not_supported)
     {
       MetaGpuKmsFlipClosureContainer *closure_container;