There is only one device mutex to lock vb2_queue when running
multi-instance encoding, it can be set by each encoder context.

[hverkuil: fix q_mutex documentation in the header]
Signed-off-by: Irui Wang <irui.wang@mediatek.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>