drm/omap: dss: Add for_each_dss_output() macro

Similarly to for_each_dss_display(), the for_each_dss_output() macro
iterates over all the DSS connected outputs.

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