vaapipostproc: add some missing locking

gst_vaapi_plugin_base_close() removed the raw caps that are used indirectly
in gst_vaapipostproc_transform_caps(). The usage is already protected by
the mutex.
This is needed when the pipeline is stopped during startup.
3 jobs for vaapipostproc-locking in 6 minutes and 19 seconds (queued for 2 seconds)