When using two monitors, displays can't turn off when inactive

## What Happened

I've setting to turn off the monitor after 5 minutes of inactivity. When using just the laptop built-in display, it works as expected, but when I've got a second monitor connected (via HDMI), it doesn't After 5 minutes, both screens turn off, but then just seconds later, they turn back on, with windows in moved to different areas of the screen.

My guess is that as one of the monitors turns off, then elementary thinks it needs to return to a single-monitor setup and the changes are interpreted as "activity", which turns everything on again. But that's just a guess without me knowing anything about anything.

## Expected Behavior

The two monitors should stay off until the computer receives input.

## Steps to Reproduce

1. Go to Settings -> Power and change "Turn off when inactive for" to 5 minutes (probably other times as well)
2. Plug an external monitor
3. Set it to extend desktop and use the external monitor as primary (can't confirm if this is required, but it is my setup)
4. Wait 5 minutes.

## Platform Information

elementary OS 5.1.7 Hera
Built on Ubuntu 18.04.4 LTS
Linux 4.15.0-122-generic
GTK 3.22.30

Dual-Core AMD A10-9600P RADEON R5, 10 COMPUTE CORES 4C+6G
Advanced Micro Devices, Inc. [AMD/ATI] Wani [Radeon R5/R6/R7 Graphics] (rev ca)