drm/dp_mst: clear time slots for ports invalid

[Why]
When change the connection status in a MST topology, mst device
which detect the event will send out CONNECTION_STATUS_NOTIFY messgae.

e.g. src-mst-mst-sst => src-mst (unplug) mst-sst

Currently, under the above case of unplugging device, ports which have
been allocated payloads and are no longer in the topology still occupy
time slots and recorded in proposed_vcpi[] of topology manager.

If we don't clean up the proposed_vcpi[], when code flow goes to try to
update payload table by calling drm_dp_update_payload_part1(), we will
fail at checking port validation due to there are ports with proposed
time slots but no longer in the mst topology. As the result of that, we
will also stop updating the DPCD payload table of down stream port.

[How]
While handling the CONNECTION_STATUS_NOTIFY message, add a detection to
see if the event indicates that a device is unplugged to an output port.
If the detection is true, then iterrate over all proposed_vcpi[] to
see whether a port of the proposed_vcpi[] is still in the topology or
not. If the port is invalid, set its num_slots to 0.

Thereafter, when try to update payload table by calling
drm_dp_update_payload_part1(), we can successfully update the DPCD
payload table of down stream port and clear the proposed_vcpi[] to NULL.

Changes since v1:(https://patchwork.kernel.org/patch/11275801/)
* Invert the conditional to reduce the indenting

Reviewed-by: Lyude Paul <lyude@redhat.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Signed-off-by: Lyude Paul <lyude@redhat.com>
[removed cc for stable - there's too many patches this depends on for
this to backport cleanly]
Link: https://patchwork.freedesktop.org/patch/msgid/20200106102158.28261-1-Wayne.Lin@amd.com

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