When running gst-discoverer-1.0, in certain media, vaapipostroc is stopped
meanwhile it is transforming caps. The problem is that stop() calls
gst_vaapi_plugin_base_close(), which nullifies the element's va display, but
the va display is used in tranform_caps() when it is extracting the possible
format conversions. This display disappearing generates warning messages.

This patch holds a local reference of va display at ensure_allowed_raw_caps()
hence it doesn't go away meanwhile it is used, even if the
gst_vaapi_plugin_base_close() is called in other thread.

https://bugzilla.gnome.org/show_bug.cgi?id=773593