media: verisilicon: av1: Fix reference video buffer pointer assignment

Always get new destination buffer for reference frame because nothing
garantees the one set previously is still valid or unused.

Fixes this chromium test suite:
https://chromium.googlesource.com/chromium/src/media/+/refs/heads/main/test/data/test-25fps.av1.ivf



Fixes: 727a4006 ("media: verisilicon: Add Rockchip AV1 decoder")
Cc: <stable@vger.kernel.org>
Signed-off-by: Benjamin Gaignard <benjamin.gaignard@collabora.com>
Reviewed-by: Nicolas Dufresne <nicolas.dufresne@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
[hverkuil: fix typo and add link to chromium test suite]