drm/i915/gtt: Tidy up ppgtt insertion for gen8

Apply the new radix shift helpers to extract the multi-level indices
cleanly when inserting pte into the gtt tree.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Abdiel Janulgue <abdiel.janulgue@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190712112725.2892-5-chris@chris-wilson.co.uk