drm/i915/gtt: No need to zero the table for page dirs

We set them to scratch right after allocation so prevent
useless zeroing before.

v2: atomic_t
v3: allow pdp alloc fail

Cc: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190614164350.30415-1-mika.kuoppala@linux.intel.com