drm/amd/display: Fix topology change issue in MST rehook

When topology changed and rehook up MST display to the same DP
connector, need to take care of drm_dp_mst_port object.

Due to the topology is changed, drm_dp_mst_port and corresponding
i2c_algorithm object could be NULL in such situation.

Signed-off-by: Jerry (Fangzhi) Zuo <Jerry.Zuo@amd.com>
Reviewed-by: Roman Li <Roman.Li@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>