drm/i915: A hotfix for making aliasing PPGTT work for GVT-g

drm/i915: A hotfix for making aliasing PPGTT work for GVT-g

This patch makes PPGTT page table non-shrinkable when using aliasing PPGTT
mode. It's just a temporary solution for making GVT-g work.

Fixes: 2ce5179fe826 ("drm/i915/gtt: Free unused lower-level page tables")
Cc: Tvrtko Ursulin <tvrtko.ursulin@linux.intel.com>
Cc: Michal Winiarski <michal.winiarski@intel.com>
Cc: Michel Thierry <michel.thierry@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Zhenyu Wang <zhenyuw@linux.intel.com>
Cc: Zhiyuan Lv <zhiyuan.lv@intel.com>
Signed-off-by: Zhi Wang <zhi.a.wang@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1486559013-25251-2-git-send-email-zhi.a.wang@intel.com
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: <stable@vger.kernel.org> # v4.10
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
(cherry picked from commit e81ecb5e31db6c2a259d694738cf620d9fa70861)
Signed-off-by: Jani Nikula <jani.nikula@intel.com>