drm/i915: NULL aliasing_ppgtt on cleanup

This will allow us to carry on if we've cleaned up the PPGTT. The usage
for this is coming up - it simplifies handling a failed PPGTT init.

Signed-off-by: Ben Widawsky <ben@bwidawsk.net>
[danvet: Spill the secrets about failing ppgtt init.]
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>