drm/omap: Don't call .set_timings() operation recursively

Instead of calling the .set_timings() operation recursively from the
display device backwards, iterate over the devices manually in the DRM
encoder code. This moves the complexity to a single central location and
simplifies the logic in omap_dss_device drivers.

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>