drm/omap: remove unneeded prototypes

omapdss.h contains prototypes for three functions, which are also
defined in dss_features.h. Remove the extra prototypes from omapdss.h.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>