vaapipostproc: ensure that the display is available when needed

Open Michael Olbrich requested to merge mol/gstreamer-vaapi:postproc-display into master

I'm not sure if this is the correct fix. But in both cases the display was not set when needed.

I could reproduce this by adding and removing a postproc element to a tee in a running pipeline quickly in a loop. Both cases happen during caps negotiation triggered by the reconfigure event.

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