When I was folding (on Linux; my situation may be different than yours), the onboard HDMI/DVI port would shut down.
If I enabled the IGP, FAHControl would complain, because in slot 0 it would find a GPU, that it can't use.
If your CPU has no IGP, the motherboard display slots are not recommended to be used, simply because your CPU will feed your primary GPU display data, which it will send back via the PCIE port, to your motherboard.
In other words, you'll have up and download data going on at the same time, which reduces performance, and causes system overhead.
You will always want to display your screen via IGP when possible (on AMD systems with AMD IGP), and disable that IGP slot if you can in FAHControl.
Or, if AMD IGP can fold, use it; but then your display will tear (as it'll stop using double/triple buffering, and put out image in single buffering mode).
That means watching a youtube video might show tearing, and watching a 1080p/4k video might cause freezes or lags.
Instead, use the first dGPU, which will set aside like 50-100MB of RAM for the desktop (might be more for Windows), and should use between 1-10% of your GPU.
Set your screen to blank screen saver, and turn off screen after x amount of minutes.
In this state, you'll use near to 100% of your GPU.