drm/i915: Unexport i915_ppgtt_init()

As i915_ppgtt_init() is not used outside of i915_gem_gtt.c we can make
it static.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/1462443767-5194-1-git-send-email-chris@chris-wilson.co.uk
Reviewed-by: Matthew Auld <matthew.auld@intel.com>