media: staging: media: imx: imx7-media-csi: Avoid unnecessary casts

Simplify the imx7_csi_media_pipeline_set_stream() function by passing it
the imx7_csi pointer, which avoids going from subdev to entity and back
to subdev.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Acked-by: Rui Miguel Silva <rmfrfs@gmail.com>
Tested-by: Alexander Stein <alexander.stein@ew.tq-group.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>