drm/i915: Apply display workaround required according to the B-Spec.

Not known to fix any current bugs.

Signed-off-by: Eric Anholt <eric@anholt.net>
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
diff --git a/drivers/gpu/drm/i915/intel_display.c b/drivers/gpu/drm/i915/intel_display.c
index 5ab4035..c41dae5 100644
--- a/drivers/gpu/drm/i915/intel_display.c
+++ b/drivers/gpu/drm/i915/intel_display.c
@@ -5819,6 +5819,10 @@
 				   ILK_CLK_FBC);
 		}
 
+		I915_WRITE(ILK_DISPLAY_CHICKEN2,
+			   I915_READ(ILK_DISPLAY_CHICKEN2) |
+			   ILK_ELPIN_409_SELECT);
+
 		if (IS_GEN5(dev)) {
 			I915_WRITE(_3D_CHICKEN2,
 				   _3D_CHICKEN2_WM_READ_PIPELINED << 16 |