OMAPDRM: Remove manager->device references

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

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.

Signed-off-by: Archit Taneja <archit@ti.com>