media: imx: Compile imx6-media-objs only for CONFIG_VIDEO_IMX_CSI

imx6-media-objs contains a set of objects that are specific to the i.MX6
IPU-based media subsystem. They're not needed for the i.MX7 CSI. Only
compile them if CONFIG_VIDEO_IMX_CSI is selected.

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