Currently when changing the manager of an overlay, set_manager() directly
calls dispc to set the overlay's destination.

Change this to be more in line with other overlay configurations, and
this will also remove the need to have dispc clocks enabled when calling
set_manager().

A new field is added to overlay struct, "manager_changed". This is
similar to "display_changed" field in manager struct, and is used to
inform apply that the manager has changed and thus write to the
registers is needed.
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>