drm/i915/gen9: Add WaFbcTurnOffFbcWatermark

According to bspec this prevents screen corruption when fbc is
used.

v2: This workaround has a name, use it (Ville)
v3: remove bogus gen check on ilk/vlv wm path (Ville)

References: HSD#2135555, HSD#2137270, BSID#562
Cc: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1465309159-30531-25-git-send-email-mika.kuoppala@intel.com

drivers/gpu/drm/i915/intel_pm.c

@@ -69,6 +69,10 @@ static void gen9_init_clock_gating(struct drm_device *dev)
 	/* WaEnableChickenDCPR:skl,bxt,kbl */
 	I915_WRITE(GEN8_CHICKEN_DCPR_1,
 		   I915_READ(GEN8_CHICKEN_DCPR_1) | MASK_WAKEMEM);
+
+	/* WaFbcTurnOffFbcWatermark:skl,bxt,kbl */
+	I915_WRITE(DISP_ARB_CTL,
+		   I915_READ(DISP_ARB_CTL) | DISP_FBC_WM_DIS);
 }
 
 static void bxt_init_clock_gating(struct drm_device *dev)