drm/i915/lvds: Restore dithering on native modes for gen2/3
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 <711c73f64afdce07b7e38039a96d2224209e9a6c@chris-wilson.co.uk>

TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,KristFoundation/Programs,TeamVee-Kanas/android_kernel_samsung_kanas,TeamVee-Kanas/android_kernel_samsung_kanas