linux-phytec-ti

drm/i915/lvds: Restore dithering on native modes for gen2/3

A regression introduced in bee17e5 cleared the dithering bit for native
modes on gen2/3.

Bugzilla: https://bugs.launchpad.net/ubuntu/+source/linux/+bug/711568
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>