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.