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)
Status Name Job ID Coverage
  Preparation
passed manifest #51000

00:00:22

 
  Build
passed build fedora x86_64 #51001

00:02:58

 
  Full Builds
passed build android arm64 api28 #51002

00:02:59