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: NZhenyu Wang <zhenyuw@linux.intel.com>
Signed-off-by: NAdam Jackson <ajax@redhat.com>
Signed-off-by: NEric Anholt <eric@anholt.net>