drm/i915: Fix LVDS dither setting

Update bdb_lvds_options structure according to its defination in
2D driver. Then we can parse and set 'lvds_dither' bit correctly
on non-965 chips.

Signed-off-by: Li Peng <peng.li@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>