drm: Document the power requirements for DP AUX transfers

When doing DP AUX transfers there are two actors that need to be
powered in order for the DP AUX transfer to work: the DP source and
the DP sink. Commit bacbab58f09d ("drm: Mention the power state
requirement on side-channel operations") added some documentation
saying that the DP source is required to power itself up (if needed)
to do AUX transfers. However, that commit doesn't talk anything about
the DP sink.

For full fledged DP the sink isn't really a problem. It's expected
that if an external DP monitor isn't plugged in that attempting to do
AUX transfers won't work. It's also expected that if a DP monitor is
plugged in (and thus asserting HPD) then AUX transfers will work.

When we're looking at eDP, however, things are less obvious. Let's add
some documentation about expectations. Here's what we'll say:

1. We don't expect the DP AUX transfer function to power on an eDP
panel. If an eDP panel is physically connected but powered off then it
makes sense for the transfer to fail.

2. We'll document that the official way to power on a panel is via the
bridge chain, specifically by making sure that the panel's prepare
function has been called (which is called by
panel_bridge_pre_enable()). It's already specified in the kernel doc
of drm_panel_prepare() that this is the way to power the panel on and
also that after this call "it is possible to communicate with any
integrated circuitry via a command bus."

3. We'll also document that for code running in the panel driver
itself that it is legal for the panel driver to power itself up
however it wants (it doesn't need to officially call
drm_panel_pre_enable()) and then it can do AUX bus transfers. This is
currently the way that edp-panel works when it's running atop the DP
AUX bus.

NOTE: there was much discussion of all of this in response to v1 [1]
of this patch. A summary of that is:
* With the Intel i195 driver, apparently eDP panels do get powered
  up. We won't forbid this but it is expected that code that wants to
  run on a variety of platforms should ensure that the drm_panel's
  prepare() function has been called.
* There is at least a reasonable amount of agreement that the
  transfer() functions itself shouldn't be responsible for powering
  the panel. It's proposed that if we need the DP AUX dev nodes to be
  robust for eDP that the code handling the DP AUX dev nodes could
  handle powering the panel by ensuring that the panel's prepare()
  call was made. Potentially drm_dp_aux_dev_get_by_minor() could be a
  good place to do this. This is left as a future exercise. Until
  that's fixed the DP AUX dev nodes for eDP are probably best just
  used for debugging.
* If a panel could be in PSR and DP AUX via the dev node needs to be
  reliable then we need to be able to pull the panel out of PSR. On
  i915 this is also apparently handled as part of the transfer()
  function.

[1] https://lore.kernel.org/r/20220503162033.1.Ia8651894026707e4fa61267da944ff739610d180@changeid

Signed-off-by: Douglas Anderson <dianders@chromium.org>
Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220509161733.v2.1.Ia8651894026707e4fa61267da944ff739610d180@changeid

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