drm/dp_mst: Enable registration of AUX devices for MST ports

All available downstream ports - physical and logical - are exposed for
each MST device. They are listed in /dev/, following the same naming
scheme as SST devices by appending an incremental ID.

Although all downstream ports are exposed, only some will work as
expected. Consider the following topology:

               +---------+
               |  ASIC   |
               +---------+
              Conn-0|
                    |
               +----v----+
          +----| MST HUB |----+
          |    +---------+    |
          |                   |
          |Port-1       Port-2|
    +-----v-----+       +-----v-----+
    |  MST      |       |  SST      |
    |  Display  |       |  Display  |
    +-----------+       +-----------+
          |Port-1
          x

 MST Path  | MST Device
 ----------+----------------------------------
 sst:0     | MST Hub
 mst:0-1   | MST Display
 mst:0-1-1 | MST Display's disconnected DP out
 mst:0-1-8 | MST Display's internal sink
 mst:0-2   | SST Display

On certain MST displays, the upstream physical port will ACK DPCD reads.
However, reads on the local logical port to the internal sink will
*NAK*. i.e. reading mst:0-1 ACKs, but mst:0-1-8 NAKs.

There may also be duplicates. Some displays will return the same GUID
when reading DPCD from both mst:0-1 and mst:0-1-8.

There are some device-dependent behavior as well. The MST hub used
during testing will actually *ACK* read requests on a disconnected
physical port, whereas the MST displays will NAK.

In light of these discrepancies, it's simpler to expose all downstream
ports - both physical and logical - and let the user decide what to use.

v3 changes:
* Change WARN_ON_ONCE -> DRM_ERROR on dpcd read errors
* Docstring and cosmetic fixes

v2 changes:

Moved remote aux device (un)registration to new mst connector late
register and early unregister helpers. Drivers should call these from
their own mst connector function hooks.

This is to solve an issue during driver unload, where mst connector
devices are unregistered before the remote aux devices are. In a setup
where aux devices are created as children of connector devices, the aux
device would be removed too early, and uncleanly. Doing so in
early_unregister solves this issue, as that is called before connector
unregistration.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Leo Li <sunpeng.li@amd.com>
Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Harry Wentland <harry.wentland@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190723232808.28128-3-sunpeng.li@amd.com