[media] v4l2-mem2mem: fix context removal from job queue in v4l2_m2m_streamoff

Just clearing the m2m_ctx->queue list_head will leave the m2m_dev->job_queue
in a broken state and can cause scheduling of device_runs after streamoff was
called.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Kamil Debski <k.debski@samsung.com>
Signed-off-by: Mauro Carvalho Chehab <m.chehab@samsung.com>