drm/i915: Fill unused GGTT with scratch pages for VT-d

One of the numerous VT-d workarounds we require is that the display
hardware reads past the end of the buffer triggering VT-d faults. This
is acknowledged in the code as being safe "since we fill the unused
portions of the GGTT with the scratch page". Alas, that is no longer
always true and so we trigger DMAR read faults.

Skylake also requires another workaround to avoid mixing VT-d and
unpopulated PTE, and so there we also need to ensure we fill unused
entries with the scratch page.

Reported-by: Mike Lothian <mike@fireburn.co.uk>
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=96584
Fixes: f7770bfd

 ("drm/i915: Skip clearing the GGTT on full-ppgtt systems")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: David Weinehall <david.weinehall@intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466773634-8106-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: David Weinehall <david.weinehall@intel.com>