drm: rcar-du: Store the number of CRTCs per group in the group structure

The number of CRTCs in a group is only used to implement plane
initialization for now, but is also needed to implement pre-association
of planes to CRTCs. Store it in the group structure instead of computing
it on demand.

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