media: imx: imx7-media-csi: Move (de)init from link setup to .s_stream()

There's no need to initialize the CSI every time a link is enabled (and
de-initialize it when a link is disabled). Move initialization to
.s_stream() instead.

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>