drm/i915: Use memset64() to prefill the GTT page

Take advantage of optimised memset64() instead of open coding it to
prefill the GTT pages.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20170926095353.11036-1-chris@chris-wilson.co.uk
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>