x86/early quirk: use gen6 stolen detection for VLV

We've always been able to use either method on VLV, but it appears more
recent BIOSes only support the gen6 method, so switch over to that.

References: https://bugs.freedesktop.org/show_bug.cgi?id=71370


Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>