OMAPDSS: outputs: Create a new entity called outputs

The current OMAPDSS design contains 3 software entities: Overlays, Managers and
Devices. These map to pipelines, overlay managers and the panels respectively in
hardware. One or more overlays connect to a manager to represent a composition,
the manager connects to a device(generally a display) to display the content.

The part of DSS hardware which isn't represented by any of the above entities
are interfaces/outputs that connect to an overlay manager, i.e blocks like DSI,
HDMI, VENC and so on. Currently, an overlay manager directly connects to the
display, and the output to which it is actually connected is ignored. The panel
driver of the display is responsible of calling output specific functions to
configure the output.

Adding outputs as a new software entity gives us the following benefits:

- Have exact information on the possible connections between managers and
  outputs: A manager can't connect to each and every output, there only limited
  hardware links between a manager's video port and some of the outputs.

- Remove hacks related to connecting managers and devices: Currently, default
  links between managers and devices are set in a not so clean way. Matching is
  done via comparing the device type, and the display types supported by the
  manager. This isn't sufficient to establish all the possible links between
  managers, outputs and devices in hardware.

- Make panel drivers more generic: The DSS panel drivers currently call
  interface/output specific functions to configure the hardware IP. When making
  these calls, the driver isn't actually aware of the underlying output. The
  output driver extracts information from the panel's omap_dss_device pointer
  to figure out which interface it is connected to, and then configures the
  corresponding output block. An example of this is when a DSI panel calls
  dsi functions, the dsi driver figures out whether the panel is connected
  to DSI1 or DSI2. This isn't correct, and having output as entities will
  give the panel driver the exact information on which output to configure.
  Having outputs also gives the opportunity to make panel drivers generic
  across different platforms/SoCs, this is achieved as omap specific output
  calls can be replaced by ops of a particular output type.

- Have more complex connections between managers, outputs and devices: OMAPDSS
  currently doesn't support use cases like 2 outputs connect to a single
  device. This can be achieved by extending properties of outputs to connect to
  more managers or devices.

- Represent writeback as an output: The writeback pipeline fits well in OMAPDSS
  as compared to overlays, managers or devices.

Add a new struct to represent outputs. An output struct holds pointers to the
manager and device structs to which it is connected. Add functions which can
register/unregister an output, or look for one. Create an enum which represent
each output instance.

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

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