drm/i915: Replace open-coded memset_p()

When initialising the page directories, we set the GTT entries and the
tree to the scratch page. We have already replaced the DMA fill with
memset64(), but we can similarly use memset_p() to set the pointer array.

References: 4dd504f7

 ("drm/i915: Use memset64() to prefill the GTT page")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180212133118.16443-1-chris@chris-wilson.co.uk