OMAPDSS: pass ovl manager to dss_start_update

dss_start_update() takes currently the dss device as a parameter. Change
the parameter to ovl manager, as that is what the dss_start_update()
actually needs. Change the name of the function to
dss_mgr_start_update() to reflect the change.

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