drm/i915/gtt: Use optimised memset32/64 for clearing PTE

Replace the open-coded memset loops with the memset32/64 routines that
reduce to a single instruction or two:

add/remove: 0/0 grow/shrink: 0/2 up/down: 0/-83 (-83)
Function                                     old     new   delta
gen6_ppgtt_clear_range                       371     344     -27
gen8_ppgtt_clear_pd                          575     519     -56

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Matthew Auld <matthew.auld@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190304230646.23714-1-chris@chris-wilson.co.uk