There are two Dell U2718Q 4K monitors connected to a Sapphire Pulse RX 570 ITX
card, one using the DisplayPort output and the other using the HDMI output.

There is a visible flickering artifact on both displays. There is no problem
when only a single display is connected.

Searching online yields a couple of results that suggest a workaround: manually
increasing the shader and/or memory clock of the GPU. On the RX 570 the
following commands can get rid of the problem:

echo "manual" > /sys/class/drm/card0/device/power_dpm_force_performance_level
echo "2" > /sys/class/drm/card0/device/pp_dpm_mclk
echo "5 6 7" > /sys/class/drm/card0/device/pp_dpm_sclk

However, I believe the amdgpu driver should be able to figure out the correct
necessary clock settings on its own. The problem exists in Linux 4.19 and
5.0-rc1 as well.