drm/fb-cma-helper: Add drm_fbdev_cma_set_suspend_unlocked()

Add a CMA version of drm_fb_helper_set_suspend_unlocked().

Cc: laurent.pinchart@ideasonboard.com
Signed-off-by: Noralf Trønnes <noralf@tronnes.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170122181117.8210-2-noralf@tronnes.org