[Why]

This patch is trying to address the issue observed when hotplug DP
daisy chain monitors.

e.g.
src-mstb-mstb-sst -> src (unplug) mstb-mstb-sst -> src-mstb-mstb-sst
(plug in again)

Once unplug a DP MST capable device, driver will call
drm_dp_mst_topology_mgr_set_mst() to disable MST. In this function,
it cleans data of topology manager while disabling mst_state. However,
it doesn't clean up the proposed_vcpis of topology manager.
If proposed_vcpi is not reset, once plug in MST daisy chain monitors
later, code will fail at checking port validation while trying to
allocate payloads.

When MST capable device is plugged in again and try to allocate
payloads by calling drm_dp_update_payload_part1(), this
function will iterate over all proposed virtual channels to see if
any proposed VCPI's num_slots is greater than 0. If any proposed
VCPI's num_slots is greater than 0 and the port which the
specific virtual channel directed to is not in the topology, code then
fails at the port validation. Since there are stale VCPI allocations
from the previous topology enablement in proposed_vcpi[], code will fail
at port validation and reurn EINVAL.

[How]

Clean up the data of stale proposed_vcpi[] and reset mgr->proposed_vcpis
to NULL while disabling mst in drm_dp_mst_topology_mgr_set_mst().

Changes since v1:
*Add on more details in commit message to describe the issue which the
patch is trying to fix
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
[added cc to stable]
Signed-off-by: Lyude Paul <lyude@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191205090043.7580-1-Wayne.Lin@amd.com
Cc: <stable@vger.kernel.org> # v3.17+