Okay so the HDA controller is bound to a driver and is runtime active. 
Naturally, if it's runtime active it'll keep the GPU awake.  Question is what's
keeping it active.

Could you check if there are any user space processes accessing the HDA
controller:
sudo lsof /dev/snd/controlC1

You got "No such file or directory" for some of the commands I gave you because
the kernel isn't compiled with CONFIG_PM_ADVANCED_DEBUG=y.  Could you enable
that option and try again?  Thanks!