drm/i915: Stop skipping the final clflush back to system pages

When we release the shmem backing storage, we make sure that the pages
are coherent with the cpu cache. However, our clflush routine was
skipping the flush as the object had no pages at release time. Fix this by
explicitly flushing the sg_table we are decoupling.

Fixes: 03ac84f1 ("drm/i915: Pass around sg_table to get_pages/put_pages backend")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/20161111145809.9701-2-chris@chris-wilson.co.uk