The issues caused by:

commit 64e62bdf ("drm/dp_mst: Remove VCPI while disabling topology
mgr")

Prompted me to take a closer look at how we clear the payload state in
general when disabling the topology, and it turns out there's actually
two subtle issues here.

The first is that we're not grabbing &mgr.payload_lock when clearing the
payloads in drm_dp_mst_topology_mgr_set_mst(). Seeing as the canonical
lock order is &mgr.payload_lock -> &mgr.lock (because we always want
&mgr.lock to be the inner-most lock so topology validation always
works), this makes perfect sense. It also means that -technically- there
could be racing between someone calling
drm_dp_mst_topology_mgr_set_mst() to disable the topology, along with a
modeset occurring that's modifying the payload state at the same time.

The second is the more obvious issue that Wayne Lin discovered, that
we're not clearing proposed_payloads when disabling the topology.

I actually can't see any obvious places where the racing caused by the
first issue would break something, and it could be that some of our
higher-level locks already prevent this by happenstance, but better safe
then sorry. So, let's make it so that drm_dp_mst_topology_mgr_set_mst()
first grabs &mgr.payload_lock followed by &mgr.lock so that we never
race when modifying the payload state. Then, we also clear
proposed_payloads to fix the original issue of enabling a new topology
with a dirty payload state. This doesn't clear any of the drm_dp_vcpi
structures, but those are getting destroyed along with the ports anyway.

Changes since v1:
* Use sizeof(mgr->payloads[0])/sizeof(mgr->proposed_vcpis[0]) instead -
  vsyrjala

Cc: Sean Paul <sean@poorly.run>
Cc: Wayne Lin <Wayne.Lin@amd.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Cc: stable@vger.kernel.org # v4.4+
Signed-off-by: Lyude Paul <lyude@redhat.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200122194321.14953-1-lyude@redhat.com