Making the discrete AMD GPU the default GPU in a Mux-less setup with an integrated Intel GPU in Mint 18 (Xenial)

[Ryan Ross]

Greetings,

I have a Toshiba Satellite P55T-B5340 that I am attempting to persuade 
to boot X using the discrete GPU, and only the discrete GPU. It's been a 
lot of fun, because it's muxless (no hardware switch, vgaswitcharoo 
seems only to crash it), and the GPU I want to use has no dedicated 
outputs (and so, somehow, has to steal them from the integrated GPU, 
using magic from what I can tell...). I've attempted to adapt the Nvidia 
approach, which sadly only appears to work for glxinfo; attempts to use 
"modesetting", screens, and AllowEmptyInitialConfiguration appear to 
have no (positive) effect on X, as it really wants this GPU to be hooked 
up to some form of output (or it will unload the module). On the upside, 
the discrete GPU is quite happy with the 'radeon' driver.

Additional Information:

The iGPU is an Intel HD 4600 Graphics (CPU is a i7-4710HQ).
The dGPU is an AMD/ATI Venus Pro (Radeon HD 8850M / R9 M265X).

Output from xrandr --listproviders is as follows:
Providers: number : 3
Provider 0: id: 0x6d cap: 0x9, Source Output, Sink Offload crtcs: 4 
outputs: 4 associated providers: 2 name:Intel
Provider 1: id: 0x45 cap: 0x6, Sink Output, Source Offload crtcs: 6 
outputs: 0 associated providers: 2 name:VERDE @ pci:0000:01:00.0
Provider 2: id: 0x45 cap: 0x6, Sink Output, Source Offload crtcs: 6 
outputs: 0 associated providers: 2 name:VERDE @ pci:0000:01:00.0

Pages I've  looked at, solutions I've tried:

https://wiki.archlinux.org/index.php/PRIME
https://www.mankier.com/5/xorg.conf#Outputclass_Section
http://us.download.nvidia.com/XFree86/Linux-x86/364.15/README/randr14.html

and a search through both Google and this greater mailing list for 
'discrete gpu', finding some interesting stuff, but nothing applicable.

Has anyone some magic glitter to spare, or perhaps some working 
knowledge of how I might get this beast into the proper configuration?

Many Thanks,
Ryan