media: amphion: lock and check m2m_ctx in event handler

driver needs to cancel vpu before releasing the vpu instance,
so call v4l2_m2m_ctx_release() first,
to handle the redundant event triggered after m2m_ctx is released.

lock and check m2m_ctx in the event handler.

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