OMAPDSS: APPLY: skip enable/disable if already enabled/disabled

Add checks to dss_mgr_enable, dss_mgr_disable, dss_ovl_enable,
dss_ovl_disable that check if the state is already the same as given in
the parameters, and exit if so.

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