OMAP: DSS2: Remove support for non-DISPC overlays

Remove support for non-DISPC overlays and overlay managers.

The support to possibly have non-DISPC overlays and managers was made to
make it possible to use CPU and/or sDMA to update RFBI or DSI command
mode displays. It is ok to remove the support, because:

- No one has used the feature.
- Display update without DISPC is very slow, so it is debatable if the
  update would even be usable.
- Removal cleans up code.
- If such a feature is needed later, it is better implemented outside
  omapdss driver.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Acked-by: Archit Taneja <archit@ti.com>