drm/i915: Unify aliasing ppgtt handling

With the dynamic pagetable alloc code aliasing ppgtt special-cases
where again mixed in all over the place with the low-level init code.

Extract the va preallocation and clearing again into the common code
where aliasing ppgtt gets set up.

Note that with this we don't set the size of the aliasing ppgtt to the
size of the parent ggtt address space. Which isn't required at all
since except for the ppgtt setup/cleanup code no one ever looks at
this.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>