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

The .check_timings() operation is called recursively from the display
device back to the output device. Most components just forward the
operation to the previous component in the chain, resulting in lots of
duplicated pass-through functions. To avoid that, iterate over the
components manually.

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>