drm/i915/gtt: Markup i915_ppgtt height

This will be useful to consolidate recursive code.

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/20190712094327.24437-3-chris@chris-wilson.co.uk