The device is not present in under the ACPI provided by Apple, I haven't had the chance to dig into a MacPro5,1 to check how the system states the device under the tree, but since there is nothing, we can just create it!
If you look under device RP08 in DSDT, you'll notice there is a device called D07E with address 0xff, this device needs to be "disabled" by means of ACPI patching, thus the "zero" scope value for D07E.
Just another way of calling the DSM method
You didn't, information is scarce about ACPI table patching (there is no general guide unfortunately) so we need to push it a little by adding relevant or wanted information to the table so it can be shown properly o as we want.
I wish there was more info about this elsewhere.. I'm afraid all the knowledge I have acquired has been through years of trial and error. One of the best sources of info about patching and such is
@RehabMan, most things I know are through "experiments" I have done based on his work and other stuff I find around.
He has a very good guide on laptop patching that englobes many aspects of ACPI patching, I'd start there.
@kgp has done tons of work in this matter too, so his post is like a little big guide
Glad I could help!