drm/i915: Flush the PTEs after updating them before suspend

drm/i915: Flush the PTEs after updating them before suspend

As we use WC updates of the PTE, we are responsible for notifying the
hardware when to flush its TLBs. Do so after we zap all the PTEs before
suspend (and the BIOS tries to read our GTT).

Fixes a regression from

commit f6996cc8b527ca42fbf1b8a1193a0a9823a9b4cf
Author: Ben Widawsky <benjamin.widawsky@intel.com>
Date:   Wed Oct 16 09:21:30 2013 -0700

    drm/i915: Disable GGTT PTEs on GEN6+ suspend

that survived and continue to cause harm even after

commit d015782ddb781071cf6347edbc10316ca77c14a0
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Wed Mar 26 20:08:20 2014 +0100

    drm/i915: Undo gtt scratch pte unmapping again

v2: Trivial rebase.
v3: Fixes requires pointer dances.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=82340
Tested-by: ming.yao@intel.com
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@vger.kernel.org
Cc: Takashi Iwai <tiwai@suse.de>
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Todd Previte <tprevite@gmail.com>
Cc: Daniel Vetter <daniel.vetter@ffwll.ch>
Reviewed-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Jani Nikula <jani.nikula@intel.com>