libs: decoder: release VA buffers after vaEndPicture
libs: decoder: release VA buffers after vaEndPicture

This change is due a problem decoding JPEGs with Intel's media-driver:
no image was generated.

This patch relases the VA buffers after vaEndPicture() is called,
and not before (after vaRenderPicture()).

https://bugzilla.gnome.org/show_bug.cgi?id=796505

ceyusa/gstreamer-vaapi,ceyusa/gstreamer-vaapi,GStreamer/gstreamer-vaapi,GStreamer/gstreamer-vaapi