drm/i915: Fix 945GM regression in e259bef

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

drivers/gpu/drm/i915/i915_gem.c

@@ -2400,7 +2400,7 @@ i915_gem_clear_fence_reg(struct drm_gem_object *obj)
 		I915_WRITE64(FENCE_REG_965_0 + (obj_priv->fence_reg * 8), 0);
 		break;
 	case 3:
-		if (obj_priv->fence_reg > 8)
+		if (obj_priv->fence_reg >= 8)
 			fence_reg = FENCE_REG_945_8 + (obj_priv->fence_reg - 8) * 4;
 		else
 	case 2: