drm/i915/gtt: Refactor common ppgtt initialisation

The basic setup of the i915_hw_ppgtt is the same between gen6 and gen8,
so refactor that into a common routine.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Bob Paauwe <bob.j.paauwe@intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190314223839.28258-5-chris@chris-wilson.co.uk