drm/i915/gtt: Push allocation to hw ppgtt constructor

In the next patch, we will subclass the gen6 hw_ppgtt. In order, for the
two different generations of hw ppgtt stucts to be of different size,
push the allocation down to the constructor.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Cc: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Cc: Matthew Auld <matthew.william.auld@gmail.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180607163040.9781-1-chris@chris-wilson.co.uk