media: imx: imx7-media-csi: Move CSI configuration before source start

There's no reason to delay the CSI configuration and the DMA setup after
starting the source. Move it before, simplifying error handling at
stream start.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Rui Miguel Silva <rmfrfs@gmail.com>
Signed-off-by: Hans Verkuil <hverkuil-cisco@xs4all.nl>
Signed-off-by: Mauro Carvalho Chehab <mchehab+huawei@kernel.org>