media: rpivid: Ensure IRQs have completed before uniniting context

Before uniniting the decode context sync with the IRQ queues to ensure
that decode no longer has any buffers in use.  This fixes a problem that
manifested as ffmpeg leaking CMA buffers when it did a stream off on
OUTPUT before CAPTURE, though in reality it was probably much more
dangerous than that.

Signed-off-by: John Cox <jc@kynesim.co.uk>