drm: omapdrm: dss: Don't export functions internal to omapdss-base

A few functions defined in omapdss-base are internal to the module.
Don't export them.

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>