drm/i915: Don't special case teardown of aliasing_ppgtt

The aliasing_ppgtt is a regular ppgtt, and we can use the regular
i915_ppgtt_put() to properly tear it down.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170215084357.19977-4-chris@chris-wilson.co.uk