drm: rcar-du: Link CRTCs to the DU device

The rcar_du_crtc functions have a heavy reliance on the rcar_du_group
structure, in many cases just to access the DU device context.

To better separate the groups out of the CRTC handling code, give the
rcar_du_crtc its own pointer to the device and remove the indirection
through the group pointers.

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