drm/i915: Use existing function instead of open-coding fence reg clear.

This is once less place to miss a new INTEL_INFO(dev)->gen update now.

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