OMAPDSS: make a two dss feat funcs internal to omapdss

dss_feat_get_supported_displays() and dss_feat_get_supported_outputs()
are not used outside omapdss, but are exported. We can thus remove the
export and move the declarations to the omapdss internal header.

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