media: mediatek: vcodec: using empty lat buffer as the last one

Adding one empty lat buffer with parameter 'is_empty_flag = true'
used to flush core work queue decode.

Queue the empty lat buffer to core list when driver need to flush decode.
It's mean core already decode all existed lat buffer when get empty lat
buffer, then wake up core decode done event, the driver will exit when
getting core dec done event.

Fixes: d227af84 ("media: mediatek: vcodec: add core decode done event")
Signed-off-by: Yunfei Dong <yunfei.dong@mediatek.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>