DP 1.2 MST does not work.
# lspci -nn | grep "VGA\|Display"
06:00.0 VGA compatible controller : Advanced Micro Devices, Inc. [AMD/ATI] Oland [Radeon HD 8570 / R7 240/340 OEM] [1002:6611] (rev 87)
# uname -a
Linux ASSET-1178 3.10.0-693.21.1.el7.x86_64 #1 SMP Wed Mar 7 19:03:37 UTC 2018 x86_64 x86_64 x86_64 GNU/Linux
# cat /etc/redhat-release
CentOS Linux release 7.4.1708 (Core)
Downloaded and installed 18.10 for CentOS.
I have spoken to Dell and 100% DP1.2 MST (DisplayPort daisy chaining) with the AMD Radeon HD 8570 / R7 240/340 OEM graphics card DOES NOT work Centos 7.4. As the graphics card only has one VGA and one DisplayPort dual monitors CANNOT be used. This is very disappointing on both the Dell and AMD front.
I should note Intel based graphics cards has DP 1.2 MST working with CentOS 7.4.
I know this post is super old but on the off chance that you still have an account with the forum...were you able to fix it? I think my problem may be a related to yours in a small way. I posted about my problem on the Dell forum here