drm/i915: Fix typo for wrong LVDS clock setting on IGDNG

New register for PCH LVDS on IGDNG should be used.
This is a copy-n-paste typo. This fixes possible dual
channel LVDS panel failure on IGDNG.

Cc: Stable Team <stable@kernel.org>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Jesse Barnes <jbarnes@virtuousgeek.org>