drm/i915: restrict vt-d stolen memory workaround to pre-gen8

drm/i915: restrict vt-d stolen memory workaround to pre-gen8

We want future generations to at least attempt to use all features, so
restrict the stolen memory disabling when vt-d is enabled to the
latest generation we have reports for. Which is a HSW per the original
report.

Also once we get a bit a hold of some of the mysterious framebuffer in
stolen memory issues that still haunt bugzilla, we should probably
drop this hack again and see what happens.

This was introduced in

commit 53565c43d2f873234180b2c1be19797606b827a2
Author: Chris Wilson <chris@chris-wilson.co.uk>
Date:   Tue Mar 18 14:50:50 2014 +0200

    drm/i915: Disable stolen memory when DMAR is active

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Jani Nikula <jani.nikula@intel.com>
Cc: David Woodhouse <dwmw2@infradead.org>
References: https://bugs.freedesktop.org/show_bug.cgi?id=68535
Acked-by: Jani Nikula <jani.nikula@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>