drm/i915: Detect virtual south bridge

Virtualized systems often use a virtual P2X4 south bridge.
Detect this in intel_detect_pch and make a best guess as to which PCH
we should be using.

This was seen on vmware esxi hypervisor. When passing the graphics device
through to a guest, it can not pass through the PCH. Instead it simulates
a P2X4 southbridge.

Signed-off-by: Robert Beckett <robert.beckett@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>