obj->framebuffer_references isn't an atomic_t so the decrement needs to
be protected by some lock. struct_mutex seems like the appropriate lock
here, and we may already take it for the obj unref anyway.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>