drm/i915: Clear fence register on tiling stride change.

The fence register value also depends upon the stride of the object, so we
need to clear the fence if that is changed as well.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
[anholt: Added 8xx and 965 paths, and renamed the confusing
i915_gem_object_tiling_ok function to i915_gem_object_fence_offset_ok]
Signed-off-by: Eric Anholt <eric@anholt.net>