Comment 2 for bug 1768003

With this GRUB in /etc/default/grub: GRUB_CMDLINE_LINUX_DEFAULT="nouveau.modeset=0"

And script:

#!/bin/bash
echo "Switching off nvidia"
# move from Nvidia to intel graphics (if not already done)
prime-select intel
# switch of power to Nvidia (repeat after each laptop start)
echo "auto" > /sys/bus/pci/devices/0000\:01\:00.0/power/control

It Nvidia powers down on my Dell XPS15 9560