drm/i915: Enable EI mode for RCx decision making on Sandybridge

And no I have no idea what the difference is either, just that is the
recommendation.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>

drivers/gpu/drm/i915/intel_display.c

@@ -6141,6 +6141,7 @@ static void gen6_enable_rc6(struct drm_i915_private *dev_priv)
 	I915_WRITE(GEN6_RC_CONTROL,
 		   GEN6_RC_CTL_RC6p_ENABLE |
 		   GEN6_RC_CTL_RC6_ENABLE |
+		   GEN6_RC_CTL_EI_MODE(1) |
 		   GEN6_RC_CTL_HW_ENABLE);
 
 	I915_WRITE(GEN6_RC_NORMAL_FREQ,