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>

drivers/gpu/drm/omapdrm/dss/dss.h

@@ -356,7 +356,6 @@ void dispc_disable_sidle(void);
 void dispc_lcd_enable_signal(bool enable);
 void dispc_pck_free_enable(bool enable);
 void dispc_enable_fifomerge(bool enable);
-void dispc_enable_gamma_table(bool enable);
 
 typedef bool (*dispc_div_calc_func)(int lckd, int pckd, unsigned long lck,
 		unsigned long pck, void *data);