media: ipu6: Don't check pipeline in stream_start

media: ipu6: Don't check pipeline in stream_start

A pipeline exists when start_streaming has returned so the check for
start_streaming_called is equivalent to having media_pipeline. Use
vb2_start_streaming_called() to perform the check.

Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Reviewed-by: Bingbu Cao <bingbu.cao@intel.com>
Tested-by: Mehdi Djait <mehdi.djait@linux.intel.com> # Dell XPS 9315
Reviewed-by: Mehdi Djait <mehdi.djait@linux.intel.com>
Signed-off-by: Hans Verkuil <hverkuil+cisco@kernel.org>