drm/i915: more virtual south bridge detection

Commit "30c964a6

 drm/i915: Detect virtual south bridge" detects and
handles the southbridge emulated by vmware esx.  Add the ich9 south
bridge emulated by 'qemu -M q35'.

Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>