Need to make sure core decode done before current instance is free.

Fixes: 365e4ba0 ("media: mtk-vcodec: Add work queue for core hardware decode")
Signed-off-by: NYunfei Dong <yunfei.dong@mediatek.com>
Reviewed-by: NAngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: NHans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: NMauro Carvalho Chehab <mchehab@kernel.org>