drm/i915/selftests: Use full release for local ppgtt allocation

We can now use the full release mechanism (i915_ppgtt_put) for our local
ppgtt allocation in igt_ppgtt_alloc.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180705065653.20449-2-chris@chris-wilson.co.uk