drm/i915: Unify SLICE_UNIT_LEVEL_CLKGATE w/a for cnl

gem_workarounds reports that the SLICE_UNIT_LEVEL_CLKGATE write isn't
sticking. Commit 0a60797a0efb ("drm/i915: Implement
ReadHitWriteOnlyDisable.") presumes that SLICE_UNIT_LEVEL_CLKGATE is a
masked register in the context image, but commit 90007bca6162
("drm/i915/cnl: Introduce initial Cannonlake Workarounds.") lists it as
an ordering unmasked register. The masked write will be losing the
default settings if we trust the original commit. That gem_workarounds
reports the value is lost entirely is more worrying though -- but it
clearly suggests that it is not a masked register in the context image,
so unify both w/a to use the original rmw.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=103705
Fixes: 0a60797a0efb ("drm/i915: Implement ReadHitWriteOnlyDisable.")
References: 90007bca6162 ("drm/i915/cnl: Introduce initial Cannonlake Workarounds.")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Rafael Antognolli <rafael.antognolli@intel.com>
Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Cc: Oscar Mateo <oscar.mateo@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20171111100336.11020-1-chris@chris-wilson.co.uk
Reviewed-by: Rafael Antognolli <rafael.antognolli@intel.com>

drivers/gpu/drm/i915/intel_engine_cs.c

@@ -1326,9 +1326,6 @@ static int cnl_init_workarounds(struct intel_engine_cs *engine)
 	WA_SET_FIELD_MASKED(GEN8_CS_CHICKEN1, GEN9_PREEMPT_GPGPU_LEVEL_MASK,
 			    GEN9_PREEMPT_GPGPU_COMMAND_LEVEL);
 
-	/* ReadHitWriteOnlyDisable: cnl */
-	WA_SET_BIT_MASKED(SLICE_UNIT_LEVEL_CLKGATE, RCCUNIT_CLKGATE_DIS);
-
 	/* WaEnablePreemptionGranularityControlByUMD:cnl */
 	I915_WRITE(GEN7_FF_SLICE_CS_CHICKEN1,
 		   _MASKED_BIT_ENABLE(GEN9_FFSC_PERCTX_PREEMPT_CTRL));

drivers/gpu/drm/i915/intel_pm.c

@@ -8471,11 +8471,13 @@ static void cnl_init_clock_gating(struct drm_i915_private *dev_priv)
 	I915_WRITE(DISP_ARB_CTL, I915_READ(DISP_ARB_CTL) |
 		   DISP_FBC_MEMORY_WAKE);
 
+	val = I915_READ(SLICE_UNIT_LEVEL_CLKGATE);
+	/* ReadHitWriteOnlyDisable:cnl */
+	val |= RCCUNIT_CLKGATE_DIS;
 	/* WaSarbUnitClockGatingDisable:cnl (pre-prod) */
 	if (IS_CNL_REVID(dev_priv, CNL_REVID_A0, CNL_REVID_B0))
-		I915_WRITE(SLICE_UNIT_LEVEL_CLKGATE,
-			   I915_READ(SLICE_UNIT_LEVEL_CLKGATE) |
-			   SARBUNIT_CLKGATE_DIS);
+		val |= SARBUNIT_CLKGATE_DIS;
+	I915_WRITE(SLICE_UNIT_LEVEL_CLKGATE, val);
 
 	/* Display WA #1133: WaFbcSkipSegments:cnl */
 	val = I915_READ(ILK_DPFC_CHICKEN);