Revert "drm/i915: Don't set the 8to6 dither flag when not scaling"

This reverts commit 773875bf

.

It is very much needed and the lack of dithering has been reported by
a large list of people with various gen2/3 hardware.

Also, the original patch was complete non-sense since the WARNING
backtraces in the references bugzilla are about
gmch_pfit.lvds_border_bits mismatch, not at all about the dither bit.
That one seems to work.

Cc: Jiri Kosina <jkosina@suse.cz>
Cc: Pavel Machek <pavel@ucw.cz>
Cc: Hans de Bruin <jmdebruin@xmsnet.nl>
Cc: stable@vger.kernel.org
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>