drm/i915: fix panel fitting filter coefficient select for Ironlake

Must set filter selection as hardcoded coefficients for medium 3x3
filtering, which matches vbios setting for Ironlake.

This fixes display corrupt issue on HP arrandale with new vbios.

Cc: Stable Team <stable@kernel.org>
Signed-off-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>