media: hantro: h264: Get the correct fallback reference buffer

If the bitstream and the application are incorrectly configuring
the reference pictures, the hardware will need to fallback
to using some other reference picture.

When the post-processor is enabled, the fallback buffer
should be a shadow buffer (postproc.dec_q), and not a
CAPTURE queue buffer, since the latter is post-processed
and not really the output of the decoder core.

Fixes: 8c2d66b0 ("media: hantro: Support color conversion via post-processing")
Signed-off-by: Ezequiel Garcia <ezequiel@collabora.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>