drm: Introduce the DP AUX bus

Historically "simple" eDP panels have been handled by panel-simple
which is a basic platform_device. In the device tree, the panel node
was at the top level and not connected to anything else.

Let's change it so that, instead, panels can be represented as being
children of the "DP AUX bus". Essentially we're saying that the
hierarchy that we're going to represent is the "control" connections
between devices. The DP AUX bus is a control bus provided by an eDP
controller (the parent) and consumed by a device like a panel (the
child).

The primary incentive here is to cleanly provide the panel driver the
ability to communicate over the AUX bus while handling lifetime issues
properly. The panel driver may want the AUX bus for controlling the
backlight or querying the panel's EDID.

The idea for this bus's design was hashed out over IRC [1].

[1] https://people.freedesktop.org/~cbrill/dri-log/?channel=dri-devel&date=2021-05-11

Cc: Laurent Pinchart <[email protected]>
Cc: Lyude Paul <[email protected]>
Cc: Rajeev Nandan <[email protected]>
Suggested-by: Laurent Pinchart <[email protected]>
Signed-off-by: Douglas Anderson <[email protected]>
Acked-by: Linus Walleij <[email protected]>
Reviewed-by: Lyude Paul <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/20210611101711.v10.4.I787c9ba09ed5ce12500326ded73a4f7c9265b1b3@changeid

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