With hw.dmar.enable=1, this Haswell laptop doesn't wake up from S3, similar symptoms to what it was doing when the TPM was enabled: no reaction to lid open / power button press, power LED keeps blinking.
Same on a Kaby Lake laptop, I guess the dmar driver just doesn't handle suspend-resume correctly.
Actually, I recall vmm.ko also breaking suspend.
The only suspend method I see in sys/amd64/vmm is in AMD ivrs_drv.c.
sys/x86/iommu/intel_drv.c has suspend/resume methods but they're blank. That explains things.