drm/i915: Use spatio-temporal dithering on PCH

Spatial dither is better than nothing, but ST is even better.

(from ajax's followup message:)
  I noticed this with:

  http://ajax.fedorapeople.org/YellowFlower.jpg

  set as my desktop background in Gnome on a 1280x800 machine (in
  particular, a Sony Vaio VPCB1 with 6-bit panel and a rather bright black
  level).  Easiest way to test this is by poking at PIPEACONF with
  intel_reg_write directly:

  % sudo intel_reg_write 0x70008 0xc0000040 # no dither
  % sudo intel_reg_write 0x70008 0xc0000050 # spatial
  % sudo intel_reg_write 0x70008 0xc0000054 # ST

  I notice it especially strongly in the relatively flat dark area in the
  top left.  Closer than about 18" I can see a noticeable checkerboard
  pattern with plain spatial dithering.  ST smooths that out; I can still
  tell that it's lacking color precision, but it's not offensive.
Signed-off-by: NAdam Jackson <ajax@redhat.com>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_reg.h

@@ -1922,7 +1922,10 @@
 /* Display & cursor control */
 
 /* dithering flag on Ironlake */
-#define PIPE_ENABLE_DITHER	(1 << 4)
+#define PIPE_ENABLE_DITHER		(1 << 4)
+#define PIPE_DITHER_TYPE_MASK		(3 << 2)
+#define PIPE_DITHER_TYPE_SPATIAL	(0 << 2)
+#define PIPE_DITHER_TYPE_ST01		(1 << 2)
 /* Pipe A */
 #define PIPEADSL		0x70000
 #define PIPEACONF		0x70008

drivers/gpu/drm/i915/intel_display.c

@@ -3677,14 +3677,16 @@ static int intel_crtc_mode_set(struct drm_crtc *crtc,
 		/* set the dithering flag */
 		if (IS_I965G(dev)) {
 			if (dev_priv->lvds_dither) {
-				if (HAS_PCH_SPLIT(dev))
+				if (HAS_PCH_SPLIT(dev)) {
 					pipeconf |= PIPE_ENABLE_DITHER;
-				else
+					pipeconf |= PIPE_DITHER_TYPE_ST01;
+				} else
 					lvds |= LVDS_ENABLE_DITHER;
 			} else {
-				if (HAS_PCH_SPLIT(dev))
+				if (HAS_PCH_SPLIT(dev)) {
 					pipeconf &= ~PIPE_ENABLE_DITHER;
-				else
+					pipeconf &= ~PIPE_DITHER_TYPE_MASK;
+				} else
 					lvds &= ~LVDS_ENABLE_DITHER;
 			}
 		}