Thank you for joining the Intel communities.
I was searching for more information to help you better but I did not find anything, honestly this is beyond out of our scope so I will try to investigate more and give you a better answer.
Have determined bit17 for the CAP register is optional in terms of how the vendor will configure BIOS
If set, will disable PM (entirely).
I tested this with FreeBSD Live CD on an HP Gen8 mircoServer with Intel C204 chipset.
FreeBSD reports port multiplier supported in AHCI and non AHCI mode.
I do not believe even so, Intel RST drivers and software support PM.
I do understand the OS offerings of other vendors may.
Thank you for that information you have kindly provided us, honestly PM is beyond out of our scope and we will not be able to help you that much here but maybe someone else can post some information that can help you any better.