OMAPDSS: modify get/find functions to go through the device chain

In the future will have arbitrarily long video pipeline chains, instead
of the current two-entities-per-pipeline model.

This patch changes the affected get/find style functions so that they
properly go through the video pipeline chain, for example when getting
the overlay manager connected to a given display.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>