drm/i915: Enable swfence debugobject support for i915.ko

Only once the debugobject symbols are exported can we enable support for
debugging swfences when i915 is built as a module. Requires commit
2617fdca3f68 ("lib/debugobjects: export for use in modules")

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/20161205142941.21965-5-chris@chris-wilson.co.uk