drm/i915/bxt: fix WaForceContextSaveRestoreNonCoherent on steppings B0+

On B0 and C0 steppings the workaround enable bit would be overriden by
default, so the overriding must be disabled.

The WA was added in
commit 83a24979c40ebbf0fa0cd14df16f74142f373cd3
Author: Nick Hoath <nicholas.hoath@intel.com>
Date:   Fri Apr 10 13:12:26 2015 +0100

    drm/i915/bxt: Add WaForceContextSaveRestoreNonCoherent

Spotted-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
diff --git a/drivers/gpu/drm/i915/i915_reg.h b/drivers/gpu/drm/i915/i915_reg.h
index 04b9f2b..3f94f38 100644
--- a/drivers/gpu/drm/i915/i915_reg.h
+++ b/drivers/gpu/drm/i915/i915_reg.h
@@ -5791,6 +5791,7 @@
 
 /* GEN8 chicken */
 #define HDC_CHICKEN0				0x7300
+#define  HDC_FORCE_CSR_NON_COHERENT_OVR_DISABLE	(1<<15)
 #define  HDC_FENCE_DEST_SLM_DISABLE		(1<<14)
 #define  HDC_DONOT_FETCH_MEM_WHEN_MASKED	(1<<11)
 #define  HDC_FORCE_CONTEXT_SAVE_RESTORE_NON_COHERENT	(1<<5)