drm/i915: refine qemu south bridge detection

drm/i915: refine qemu south bridge detection

The test for the qemu q35 south bridge added by commit
"39bfcd52 drm/i915: more virtual south bridge detection"
also matches on real hardware.  Having the check for
virtual systems last in the list is not enough to avoid
that ...

Refine the check by additionally verifying the pci
subsystem id to see whenever it *really* is qemu.

[ v2: fix subvendor tyops ]

Reported-and-tested-by: Bjørn Mork <bjorn@mork.no>
Signed-off-by: Gerd Hoffmann <kraxel@redhat.com>
Tested-by: Bruno Wolff III <bruno@wolff.to>
Cc: drm-intel-fixes@lists.freedesktop.org
Signed-off-by: Jani Nikula <jani.nikula@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1453719748-10944-1-git-send-email-kraxel@redhat.com
(cherry picked from commit 1e859111c128265f8d62b39ff322e42b1ddb5a20)