drm/i915/selftests: Replace open-coded i915_address_space_init()

Use i915_address_space_init() rather than open-code it inside
mock_ppgtt() as we will forget to keep it in sync.

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/20180705065653.20449-3-chris@chris-wilson.co.uk