drm/i915: Move aliasing_ppgtt underneath its i915_ggtt

The aliasing_ppgtt provides a PIN_USER alias for the global gtt, so move
it under the i915_ggtt to simplify later transformations to enable
intel_context.vm.

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/20190730143209.4549-1-chris@chris-wilson.co.uk