OMAPDSS: register only one display device per output

OMAPDSS: register only one display device per output

We have boards with multiple panel devices connected to the same
physical output, of which only one panel can be enabled at one time.
Examples of these are Overo, where you can use different daughter boards
that have different LCDs, and 3430SDP which has an LCD and a DVI output
and a physical switch to select the active display.

These are supported by omapdss so that we add all the possible display
devices at probe, but the displays are inactive until somebody enables
one. At this point the panel driver starts using the DSS, thus reserving
the physcal resource and excluding the other panels.

This is problematic:
- Panel drivers can't allocate their resources properly at probe(),
  because the resources can be shared with other panels. Thus they can
  be only reserved at enable time.
- Managing this in omapdss is confusing. It's not natural to have
  child devices, which may not even exist (for example, a daughterboard
  that is not connected).

Only some boards have multiple displays per output, and of those, only
very few have possibility of switching the display during runtime.
Because of the above points:
- We don't want to make omapdss and all the panel drivers more complex
  just because some boards have complex setups.
- Only few boards support runtime switching, and afaik even then it's
  not required. So we don't need to support runtime switching.

Thus we'll change to a model where we will have only one display device
per output and this cannot be (currently) changed at runtime. We'll
still have the possibility to select the display from multiple options
during boot with the default display option.

This patch accomplishes the above by changing how the output drivers
register the display device. Instead of registering all the devices
given from the board file, we'll only register one. If the default
display option is set, the output driver selects that display from its
displays. If the default display is not set, or the default display is
not one of the output's displays, the output driver selects the first
display.

Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>