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>

drivers/gpu/drm/i915/i915_reg.h

@@ -5791,6 +5791,7 @@ enum skl_disp_power_wells {
 
 /* 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)

drivers/gpu/drm/i915/intel_ringbuffer.c

@@ -1043,6 +1043,7 @@ static int bxt_init_workarounds(struct intel_engine_cs *ring)
 {
 	struct drm_device *dev = ring->dev;
 	struct drm_i915_private *dev_priv = dev->dev_private;
+	uint32_t tmp;
 
 	gen9_init_workarounds(ring);
 
@@ -1058,8 +1059,10 @@ static int bxt_init_workarounds(struct intel_engine_cs *ring)
 	}
 
 	/* WaForceContextSaveRestoreNonCoherent:bxt */
-	WA_SET_BIT_MASKED(HDC_CHICKEN0,
-			  HDC_FORCE_CONTEXT_SAVE_RESTORE_NON_COHERENT);
+	tmp = HDC_FORCE_CONTEXT_SAVE_RESTORE_NON_COHERENT;
+	if (INTEL_REVID(dev) >= BXT_REVID_B0)
+		tmp |= HDC_FORCE_CSR_NON_COHERENT_OVR_DISABLE;
+	WA_SET_BIT_MASKED(HDC_CHICKEN0, tmp);
 
 	return 0;
 }