drm/i915: disable temporal dithering on the internal panel

drm/i915: disable temporal dithering on the internal panel

<ajax> i'm getting tempted to just disable temporal
<mjg59> Approved.
<ajax> apparently it makes the screen look pulse-y which is worse
than the disease.

References: http://lists.freedesktop.org/archives/intel-gfx/2011-October/012545.html
Tested-by: Олег Герман <oleg.german@gmail.com>
Reviewed-by: Adam Jackson <ajax@redhat.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Signed-off-by: Keith Packard <keithp@keithp.com>