media: hantro: postproc: Fix motion vector space allocation

When the post-processor is enabled, the driver allocates
"shadow buffers" which are used for the decoder core,
and exposes the post-processed buffers to userspace.

For this reason, extra motion vector space has to
be allocated on the shadow buffers, which the driver
wasn't doing. Fix it.

This fix should address artifacts on high profile bitstreams.

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>