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

stable inclusion
from stable-v5.10.177
commit 7051d51f1241d02a9db159485bddfe508427ab01
category: bugfix
bugzilla: https://gitee.com/openeuler/kernel/issues/I88YNP

Reference: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=7051d51f1241d02a9db159485bddfe508427ab01



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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>
Signed-off-by: sanglipeng <sanglipeng1@jd.com>