OMAPDSS: APPLY: move mgr->enabled to mgr_priv_data

struct omap_overlay_manager contains "enabled"-field, used to track if
the manager is enabled or not. This field should be internal to apply.c.

This patch moves the field to mgr_priv_data, and applies the necessary
locking when accessing the field.

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