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

Apparently we really only need this when the pfit is enabled, at least
I couldn't dicern any difference here. Furthermore the hacks we have
to reconstruct this bit is a bit glaring, and probably only works
because we can't move the lvds port to any other pipe than pipe B on
gen2/3.

So let's just rip this out.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=77137

 (the LVDS
WARNING log, not the main "VGA can't be turned on" issue).
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>