media: amphion: release m2m ctx when releasing vpu instance

release m2m ctx in the callback function that
release the vpu instance, then there is no need
to add lock around releasing m2m ctx.

Fixes: 3cd08451 ("media: amphion: add vpu v4l2 m2m support")
Signed-off-by: Ming Qian <ming.qian@nxp.com>
Reviewed-by: Tommaso Merciai <tommaso.merciai@amarulasolutions.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>