drm/i915/gtt: Reorder gen8 ppgtt free/clear/alloc

In preparation for refactoring the free/clear/alloc, first move the code
around so that we can avoid forward declarations in the next set of
patches.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712094327.24437-2-chris@chris-wilson.co.uk