media: staging/intel-ipu3: Cleanup dummy buffers via helper

Use an existing helper imgu_video_nodes_exit() on imgu-video-node
initialization failure path to cleanup dummy buffers.

Signed-off-by: Umang Jain <umang.jain@ideasonboard.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Sakari Ailus <sakari.ailus@linux.intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>