drm/amd/display: move MST branch initialize to before link training

some MST capable scaler doesn't like recieving CLEAR_PAYLOAD_ID_TABLE after
link training.  move branch initialize to before link training

Signed-off-by: Hersen Wu <hersenxs.wu@amd.com>
Signed-off-by: Tony Cheng <tony.cheng@amd.com>
Reviewed-by: Tony Cheng <Tony.Cheng@amd.com>
Acked-by: Harry Wentland <harry.wentland@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>