drm/amd/display: Fix MST reboot/poweroff sequence

[Why]

drm_dp_mst_topology_mgr_suspend() is added into the new reboot
sequence, which disables the UP request at the beginning.
Therefore sideband messages are blocked.

[How]

Finish MST sideband message transaction before UP request is
suppressed.
Signed-off-by: Leo (Hanghong) Ma <hanghong.ma@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Leo Li <sunpeng.li@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org

drivers/gpu/drm/amd/display/amdgpu_dm/amdgpu_dm.c

@@ -786,12 +786,13 @@ static int dm_suspend(void *handle)
 	struct amdgpu_display_manager *dm = &adev->dm;
 	int ret = 0;
 
+	WARN_ON(adev->dm.cached_state);
+	adev->dm.cached_state = drm_atomic_helper_suspend(adev->ddev);
+
 	s3_handle_mst(adev->ddev, true);
 
 	amdgpu_dm_irq_suspend(adev);
 
-	WARN_ON(adev->dm.cached_state);
-	adev->dm.cached_state = drm_atomic_helper_suspend(adev->ddev);
 
 	dc_set_power_state(dm->dc, DC_ACPI_CM_POWER_STATE_D3);