drm/omap: dss: Support for detecting display stack readiness

When omapdss is loaded (all core components are in place) create a list of
devices used in the display graph. This list later can be used by omapdrm
via the omapdss_stack_is_ready() function to check that these components
are loaded. Based on this information, omapdrm can defer probe in case when
the omapdss stack is not ready yet.

Signed-off-by: Peter Ujfalusi <peter.ujfalusi@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>