drm/i915: Rename i915_hw_ppgtt to i915_ppgtt

Keeping the _hw_ in there does not help to distinguish it from its
only brethren i915_ggtt, so drop it.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190611091238.15808-2-chris@chris-wilson.co.uk