drm/amd/display: use MST downstream AUX to dump DPRX CRCs

[why]
     In MST setup, we'd use MST downstream AUX to
     dump the DPRX CRCs from sink device.

[how]
     Assign the mst_port->aux to read DPCD registers.

Signed-off-by: David (Dingchen) Zhang <dingchen.zhang@amd.com>
Reviewed-by: Harry Wentland <Harry.Wentland@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>