drm/i915: Add support for fence registers on Ivybridge.

The registers are the same as on Sandybridge.  Fixes scrambled display
in X when it does software drawing to the GTT, and scans the results
out as tiled.

Signed-off-by: Eric Anholt <eric@anholt.net>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Keith Packard <keithp@keithp.com>