xrand --output ... --off no longer turns the screen off on Linux >=4.15

[WGH]

On Linux 4.14 and earlier, running `xrandr --output ${MONITOR} --off`
will result in screen turning off (the monitor will say "no signal" or so).

Since 4.15, however, the screen will remain black, but turned on
(backlight and everything).

I'm running xrandr 1.5.0 and X.Org Server 1.20.0.

This is caused by kernel change introduced here:
https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/?id=846c7dfc1193eb2f9866fe2fa0ae7d45c72f95b9

I've reported this a bug to X.Org as well. 
https://bugs.freedesktop.org/show_bug.cgi?id=107472