media: exynos4-is: Handle duplicate calls to vidioc_streamoff

vidioc_streamoff can be called multiple times from userspace, but we
should only call media_pipeline_stop when we're actually setup.

This became more noticeable after commit 2a2599c6 ("[media] media:
entity: Catch unbalanced media_pipeline_stop calls") was merged as it
added a WARN for unbalanced calls to media_pipeline_stop.

Signed-off-by: Jonathan Bakker <xc-racer2@live.ca>
Reviewed-by: Sylwester Nawrocki <s.nawrocki@samsung.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>