With the introduction of output entities, managers will now connect to outputs.
Use the helper op for overlays named get_device. This will abstract away the
information on how to get the omap_dss_device pointer from an overlay.

Using the helper function will reduce the number of pointer dereferences a user
of OMAPDSS needs to do and reduce risk of a NULL dereference.

Cc: Vaibhav Hiremath <hvaibhav@ti.com>
Signed-off-by: Archit Taneja <archit@ti.com>