OMAPDSS: Remove old way of setting manager and device links

Now that an omap_dss_output can be used to link between managers and devices, we
can remove the old way of setting manager and device links. This involves
removing the device and manager pointers from omap_overlay_manager and
omap_dss_device respectively, and removing the set_device/unset_device ops from
omap_overlay_manager.

Signed-off-by: Archit Taneja <archit@ti.com>