Fix visual artifact on Linux running AMD 3400G

I did a mini-itx build with AMD 3400G. It works great except occasionaly visual artifacts which affected Firefox the most when watching videos. The problem is solved by disabling IOMMU. I verfied that setting kernel parameter iommu=pt also works [https://bugs.launchpad.net/ubuntu/+source/xserver-xorg-video-amdgpu/+bug/1848741] . https:…