OMAPDSS: Remove manager->device references

With the introduction of output entities, managers will now connect to outputs.
Create helper ops for overlays and managers named get_device. This will abstract
away the information on how to get the device from an overlay or an overlay
manager. The get_device ops currently retrieve the output via a
ovl->manager->device reference. This will be later replaced by
ovl->manager->output->device references.

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