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