drm: Constify drm_mode_config atomic helper private pointer

The drm_mode_config helper private field points to a structure of
function pointers that don't need to be modified at runtime. Make it
const.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Gabriel Krisman Bertazi <krisman@collabora.co.uk>
Acked-by: Brian Starkey <brian.starkey@arm.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: http://patchwork.freedesktop.org/patch/msgid/20170102091613.6310-1-laurent.pinchart@ideasonboard.com