drm/i915: Add support for dual-channel LVDS on 8xx.

This corresponds to a fix to UMS back in 2007.  Fixes fd.o bug #20115.
Signed-off-by: NMa Ling <ling.ma@intel.com>
Signed-off-by: NEric Anholt <eric@anholt.net>

drivers/gpu/drm/i915/intel_display.c

@@ -90,7 +90,7 @@ struct intel_limit {
 #define I8XX_P2_SLOW		      4
 #define I8XX_P2_FAST		      2
 #define I8XX_P2_LVDS_SLOW	      14
-#define I8XX_P2_LVDS_FAST	      14 /* No fast option */
+#define I8XX_P2_LVDS_FAST	      7
 #define I8XX_P2_SLOW_LIMIT	 165000
 
 #define I9XX_DOT_MIN		  20000