OMAPDSS: fix dss_ovl_unset_manager

When we removed fifomerge support, we also changed dss_ovl_disable so
that it doesn't wait for the hardware to be finished with the overlay.
This may cause a problem when changing the overlay's manager, as
changing the manager is an immediate change. Thus if the overlay is
still being used by the HW when the manager is changed, there may be
glitches on the screen.

This patch adds a wait into dss_ovl_unset_manager, which ensures the
overlays are disabled in the HW.

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