drm/amd/display: Clear edid when unplug mst connector

[Why]
When unplug one sst monitor from a mst hub and plug in the same
port with another sst monitor, we don't read the corresponding
edid. That's because we detect there is already an edid stored in
aconnector->edid which is a stale one.

[How]
Clean up aconnector->edid when unplug mst connector.

Reviewed-by: Hersen Wu <hersen.wu@amd.com>
Acked-by: Solomon Chiu <solomon.chiu@amd.com>
Signed-off-by: Wayne Lin <Wayne.Lin@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>