omap_dss_mgr_apply currently applies settings to all overlays and
overlay managers. The reason for this was to support cases where
configuration changes affecting multiple managers are made. However, the
current code doesn't support changing such configurations, so the
functionality is not needed.

Change the apply to affect only the manager given as an argument, and
the overlays attached to that manager.
Signed-off-by: NTomi Valkeinen <tomi.valkeinen@ti.com>