drm/i915: Clear any existing dither mode prior to enabling spatial dithering

We cannot the initial configuration set by the BIOS not to have a dither
mode enabled which conflicts with our enabling the Spatial Temporal 1
dither mode for PCH. In particular, the BIOS may either enable temporal
dithering or the Spatial Temporal 2 with the result that we enable pure
temporal dithering. Temporal dithering looks bad and is perceived as a
flicker.

Fixes:

  Bug 29248 - [Arrandale] Annoying flicker on internal panel, goes away
              after suspend to RAM
  https://bugs.freedesktop.org/show_bug.cgi?id=29248

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -3736,6 +3736,7 @@ static int intel_crtc_mode_set(struct drm_crtc *crtc,
 			if (dev_priv->lvds_dither) {
 				if (HAS_PCH_SPLIT(dev)) {
 					pipeconf |= PIPE_ENABLE_DITHER;
+					pipeconf &= ~PIPE_DITHER_TYPE_MASK;
 					pipeconf |= PIPE_DITHER_TYPE_ST01;
 				} else
 					lvds |= LVDS_ENABLE_DITHER;