iommu/amd: Disable IOMMU on Stoney Ridge systems

iommu/amd: Disable IOMMU on Stoney Ridge systems

BugLink: https://bugs.launchpad.net/bugs/1864005
Serious screen flickering when Stoney Ridge outputs to a 4K monitor.

Use identity-mapping and PCI ATS doesn't help this issue.

According to Alex Deucher, IOMMU isn't enabled on Windows, so let's do
the same here to avoid screen flickering on 4K monitor.

Cc: Alex Deucher <alexander.deucher@amd.com>
Bug: https://gitlab.freedesktop.org/drm/amd/issues/961
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Acked-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Joerg Roedel <jroedel@suse.de>
(cherry picked from commit 3dfee47b215e49788cfc80e474820ea2e948c031)
Signed-off-by: Kai-Heng Feng <kai.heng.feng@canonical.com>
Signed-off-by: Seth Forshee <seth.forshee@canonical.com>