drm: omapdrm: dss: Remove unused functions prototypes

The omap_dss_register_driver(), omap_dss_unregister_driver() and
dispc_enable_gamma_table() functions don't exist anymore, remove their
prototypes.

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>