drm/i915: Fix pte updates in ggtt clear range

This bug was introduced by me:
commit e76e9aeb
Author: Ben Widawsky <ben@bwidawsk.net>
Date:   Sun Nov 4 09:21:27 2012 -0800

    drm/i915: Stop using AGP layer for GEN6+

The existing code uses memset_io which follows memset semantics in only
guaranteeing a write of individual bytes. Since a PTE entry is 4 bytes,
this can only be correct if the scratch page address is 0.

This caused unsightly errors when we clear the range at load time,
though I'm not really sure what the heck is referencing that memory
anyway. I caught this is because I believe we have some other bug where
the display is doing reads of memory we feel should be cleared (or we
are relying on scratch pages to be a specific value).

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>