drm/omap: Reverse direction of the DSS device enable/disable operations

The omapdrm and omapdss drivers are architectured based on display
pipelines made of multiple components handled from sink (display) to
source (DSS output). This is incompatible with the DRM bridge and panel
APIs that handle components from source to sink.

Reconcile the omapdrm and omapdss drivers with the DRM bridge and panel
model by reversing the direction of the DSS device .enable() and
.disable() operations. This completes the move to the DRM bridge model,
with the notable exception of the DSI pipelines that will require more
work.

We also adapt the omapdss shutdown handler dss_shutdown() to shut down
all active pipelines starting from the pipeline output device instead of
the display device.

As a consequence the for_each_dss_display() macro isn't used and can be
removed, and the omapdss_device_get_next() function underlying the macro
can be simplified to search for output devices only.

Signed-off-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Tested-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>