Comment # 1 on bug 1185970 from Takashi Iwai

First off, disabling ACPI is really bad on modern systems, and most likely it'd
break anything else.  So please don't do it unless you must do so temporarily.

About the AMDGPU: the driver doesn't support the hotplug removal, as it seems,
it spews the error message like:
  [drm:amdgpu_pci_remove [amdgpu]] *ERROR* Hotplug removal is not supported

So the way you're trying to switch the graphics is no supported operation.

Usually switching the graphics between switchable GPUs is done via
vga_switcheroo.  Or for the hybrid GPU, PRIME is used.