OMAPDSS: APPLY: fix NULL pointer deref when mgr is not set

extra_info_update_ongoing() goes through all overlays, but doesn't check
if the overlay is connected to a manager. This leads to a crash whenever
an overlay has been detached.

Add a check to skip the non-connected overlays.

Reported-by: Rob Clark <rob@ti.com>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>