drm/rcar-du: Use dynamic number of CRTCs instead of CRTCs array size

The rcar_du_device structure contains a field that stores the number of
CRTCs, use it instead of the CRTCs array size. This prepares the driver
to support a variable number of CRTCs.

Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>