drm/i915: Enable/disable the dithering for LVDS based on VBT setting

Enable/disable the dithering for LVDS based on VBT setting. On the 965/g4x
platform the dithering flag is defined in LVDS register. And on the ironlake
the dithering flag is defined in pipeconf register.

Signed-off-by: Zhao Yakui <yakui.zhao@intel.com>
Signed-off-by: Eric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/i915_reg.h

@@ -975,6 +975,8 @@
 #define   LVDS_PORT_EN			(1 << 31)
 /* Selects pipe B for LVDS data.  Must be set on pre-965. */
 #define   LVDS_PIPEB_SELECT		(1 << 30)
+/* LVDS dithering flag on 965/g4x platform */
+#define   LVDS_ENABLE_DITHER		(1 << 25)
 /* Enable border for unscaled (or aspect-scaled) display */
 #define   LVDS_BORDER_ENABLE		(1 << 15)
 /*
@@ -1744,6 +1746,8 @@
 
 /* Display & cursor control */
 
+/* dithering flag on Ironlake */
+#define PIPE_ENABLE_DITHER	(1 << 4)
 /* Pipe A */
 #define PIPEADSL		0x70000
 #define PIPEACONF		0x70008

drivers/gpu/drm/i915/intel_display.c

@@ -3195,7 +3195,20 @@ static int intel_crtc_mode_set(struct drm_crtc *crtc,
 		 * appropriately here, but we need to look more thoroughly into how
 		 * panels behave in the two modes.
 		 */
-
+		/* set the dithering flag */
+		if (IS_I965G(dev)) {
+			if (dev_priv->lvds_dither) {
+				if (IS_IRONLAKE(dev))
+					pipeconf |= PIPE_ENABLE_DITHER;
+				else
+					lvds |= LVDS_ENABLE_DITHER;
+			} else {
+				if (IS_IRONLAKE(dev))
+					pipeconf &= ~PIPE_ENABLE_DITHER;
+				else
+					lvds &= ~LVDS_ENABLE_DITHER;
+			}
+		}
 		I915_WRITE(lvds_reg, lvds);
 		I915_READ(lvds_reg);
 	}