drm/i915: Disable GGTT PTEs on GEN6+ suspend

Once the machine gets to a certain point in the suspend process, we
expect the GPU to be idle. If it is not, we might corrupt memory.
Empirically (with an early version of this patch) we have seen this is
not the case. We cannot currently explain why the latent GPU writes
occur.

In the technical sense, this patch is a workaround in that we have an
issue we can't explain, and the patch indirectly solves the issue.
However, it's really better than a workaround because we understand why
it works, and it really should be a safe thing to do in all cases.

The noticeable effect other than the debug messages would be an increase
in the suspend time. I have not measure how expensive it actually is.

I think it would be good to spend further time to root cause why we're
seeing these latent writes, but it shouldn't preclude preventing the
fallout.

NOTE: It should be safe (and makes some sense IMO) to also keep the
VALID bit unset on resume when we clear_range(). I've opted not to do
this as properly clearing those bits at some later point would be extra
work.

v2: Fix bugzilla link

Bugzilla: http://bugs.freedesktop.org/show_bug.cgi?id=65496
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=59321


Tested-by: Takashi Iwai <tiwai@suse.de>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Tested-By: Todd Previte <tprevite@gmail.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>