media: mediatek: vcodec: move core context from device to each instance

There are so many lat buffer in core context list, some instances
maybe be scheduled for a very long time. Moving the core context to
each instance, it only be used to control lat buffer of each instance.
And the core work queue of each instance is scheduled by system.

Fixes: 2cfca6c1 ("media: mediatek: vcodec: move lat_buf to the top of core list")
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>