drm/i915/skl: Initialize PPGTT like gen8

gen9 uses very similar memory management to what gen8 has. Just follow
the flow.

v2: Fix trivial conflict (Damien)

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Ben Widawsky <ben@bwidawsk.net>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>