drm: rcar-du: Share plane atomic check code between Gen2 and Gen3

The plane atomic check implementation is identical on Gen2 (DU planes)
and Gen3 (VSP planes), but two separate functions exist as they operate
on different data structures. Refactor the code to share the
implementation.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Tested-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>
Reviewed-by: Kieran Bingham <kieran.bingham+renesas@ideasonboard.com>