OMAPDSS: outputs: Create and register output instances

Add output structs to output driver's private data. Register output instances by
having an init function in the probes of the platform device drivers for
different outputs. The *_init_output for each output registers the output and
fill up the output's plaform device, type and id fields. The *_uninit_output
functions unregister the output.

In the probe of each interface driver, the output entities are initialized
before the *_probe_pdata() functions intentionally. This is done to ensure that
the output entity is prepared before the panels connected to the output are
registered. We need the output entities to be ready because OMAPDSS will try
to make connections between overlays, managers, outputs and devices during the
panel's probe.

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