OK, here is a new twist: upon further investigating this, I came across some
weird backlight behaviour that I originally reported here:
https://gitlab.gnome.org/GNOME/gnome-settings-daemon/issues/53. However, as it
turns out, this is not a gnome issue but a kernel one.

Namely, doing 

  echo X > /sys/class/backlight/intel_backlight/brightness

will wake my dGPU, even though it really shouldn't be involved at all (and does
not have a /sys/class/backlight device registered). Furthermore, repeatedly
invoking this command before the dGPU has had the chance to go back to sleep
will not do anything. Thirdly, this seems to be a very good way to get the
system to hard lock up.

So, is this an amdgpu issue? A i915 issue? ACPI? where should I take this bug
report?