drm/i915/pch: Use minimal number of FDI lanes (v2)

This should be a small power savings. Tested on Lenovo T410 (Ironlake), LVDS
VGA and DisplayPort, up to 1920x1200R.

v2: Add Sandybridge support, fix obvious math error.

Acked-by: Zhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Eric Anholt <eric@anholt.net>