drm: rcar-du: Store CMM device pointer instead of platform_device

drm: rcar-du: Store CMM device pointer instead of platform_device

The DU driver stores the CMM devices as pointers to struct
platform_device, and passes them to the API exposed by the CMM driver.
This is similar to how the VSP is handled, except that the VSP uses
struct device pointers. Replace the CMM platform_device pointers with
device pointers for consistency.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Link: https://patch.msgid.link/20260323164526.2292491-3-laurent.pinchart+renesas@ideasonboard.com
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ideasonboard.com>