drm/omap: dss: Constify omap_dss_driver operations structure

The structure contains function pointers that don't need to be modified.
Make all its instances const to improve security.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.co.uk>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>