staging: media: ipu3: Return buffers outside of needless locking

In imgu_vb2_start_streaming()'s error path, imgu_return_all_buffers()
is outside the streaming_lock and after the call to
video_device_pipeline_stop().

Let's apply the same order in imgu_vb2_stop_streaming() as well.

Signed-off-by: Max Staudt <mstaudt@chromium.org>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>