drm/i915: Remove i915_gem_object_to_ggtt()

With the last user of this convenience wrapper gone, we can kill the
wrapper and in the process make the lookup function static.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: http://patchwork.freedesktop.org/patch/msgid/20170116152131.18089-5-chris@chris-wilson.co.uk
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>