drm: rcar-du: Remove unused CRTC suspend/resume functions

An early implementation of suspend-resume helpers are available in the
CRTC module, however they are unused and no longer needed.

With suspend and resume handled by the core DRM atomic helpers, we can
remove the unused functions.

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