drm: rcar-du: Don't leak device_link to CMM

drm: rcar-du: Don't leak device_link to CMM

The DU driver creates device_link instances between the DU and CMMs, but
never deletes them. Fix it by introducing a rcar_du_cmm structure to
group the CMM device and device_link, and deleting the links at cleanup
time.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patch.msgid.link/20260323164526.2292491-5-laurent.pinchart+renesas@ideasonboard.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>