drm/fb_cma_helper: add suspend helper

Implement a suspend/resume helper for CMA users which calls
drm_fb_helper_set_suspend.

Suggested-by: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Stefan Agner <stefan@agner.ch>