vaapidecode: partially revert 0777f35.
vaapidecode: partially revert 0777f35.

Reset the VA decoder after updating the base plugin caps, and most
importantly, after GstVideoDecoder negotiation. The reason behind
this is that the negotiation could trigger a last decide_allocation()
where we could actually derive a new GstVaapiDisplay to use from the
downstream element. e.g. GLX backend.

GStreamer/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,CapOM/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi