drm/i915: Avoid potential division-by-zero in computing CS timestamp period

Since we use a HW readback or estimation of the CS timestamp frequency,
sometimes it may result in 0. Avoid the division-by-zero in computing
its reciprocal, the timestamp period.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Acked-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20200216161746.500258-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_device_info.c

@@ -1044,13 +1044,17 @@ void intel_device_info_runtime_init(struct drm_i915_private *dev_priv)
 	}
 
 	/* Initialize command stream timestamp frequency */
-	runtime->cs_timestamp_frequency_khz = read_timestamp_frequency(dev_priv);
+	runtime->cs_timestamp_frequency_khz =
+		read_timestamp_frequency(dev_priv);
+	if (runtime->cs_timestamp_frequency_khz) {
 		runtime->cs_timestamp_period_ns =
 			div_u64(1e6, runtime->cs_timestamp_frequency_khz);
 		drm_dbg(&dev_priv->drm,
 			"CS timestamp wraparound in %lldms\n",
-		div_u64(mul_u32_u32(runtime->cs_timestamp_period_ns, S32_MAX),
+			div_u64(mul_u32_u32(runtime->cs_timestamp_period_ns,
+					    S32_MAX),
 				USEC_PER_SEC));
+	}
 }
 
 void intel_driver_caps_print(const struct intel_driver_caps *caps,