OMAPDSS: fix registering the vsync isr in apply

When we enable an output we don't check if we need to register the vsync
isr. This causes us to miss vsync interrupts until somebody changes the
configuration of an overlay or an overlay manager.

Add the registration to dss_mgr_enable to fix the problem.

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