i915: only use tiled blits on 965+

When scheduled swaps occur, we need to blit between front & back buffers.  If
the buffers are tiled, we need to set the appropriate XY_SRC_COPY tile bit, but
only on 965 chips, since it will cause corruption on pre-965 (e.g. 945).

Bug reported by and fix tested by Tomas Janousek <tomi@nomi.cz>.

Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>