drm/i915: Organize PPGTT init

Let's be optimistic that for future platforms memory management doesn't change
that much and reuse gen8 function for PPGTT init.

Cc: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>