drm/i915: Move i915_gem_object_wait_rendering()

Just move it earlier so that we can use the companion nonblocking
version in a couple of more callsites without having to add a forward
declaration.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1470324762-2545-24-git-send-email-chris@chris-wilson.co.uk