drm/i915: Fix 945GM regression in e259befd

A minor typo caused a single fence register to be incorrectly
programmed, resulting in occassional tiling corruption.

Reported-and-tested-by: Hans de Bruin <bruinjm@xs4all.nl>
Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=18962
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: stable@kernel.org