media: staging: rkisp1: cap: serialize start/stop stream

In order to support simultaneous streaming from both capture devices,
start/stop vb2 calls need to be serialized to allow multiple concurrent
calls.

Signed-off-by: Helen Koike <helen.koike@collabora.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>