drm/amd/display: Add DSC Support for Synaptics Cascaded MST Hub

commit f4f3b7de

 upstream.

Traditional synaptics hub has one MST branch device without virtual dpcd.
Synaptics cascaded hub has two chained MST branch devices. DSC decoding
is performed via root MST branch device, instead of the second MST branch
device.

Reviewed-by: Hersen Wu <hersenxs.wu@amd.com>
Acked-by: Qingqing Zhuo <qingqing.zhuo@amd.com>
Signed-off-by: Fangzhi Zuo <Jerry.Zuo@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>
Cc: stable@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>