I have exactly same problem. Have not found solution yet. There are no firmwares for monitors or BIOS for VGA. No much options there.
I have found that there is that nice option changing 1.2 DP when monitors are offline. So this is the key player here.
Got my screen working in MST extended desktop mode, but it does not work together with HDMI :( I am using HDMI receiver for audio. As I am turning on my receiver both display port monitors turns off. I can enable primary DP monitor by disabling DP 1.2 with HDMI enabled, but it dies not fit me.