drm/i915: Extract aliasing ppgtt setup

In order to force testing of the aliasing ppgtt, extract its
initialisation function.

v2: Also extract the cleanup function for symmetry.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20170213171558.20942-39-chris@chris-wilson.co.uk