drm/i915: make CS frequency read support missing more obvious

As suggested by Chris, we should make this more obvious for people
working with newer generations.
Suggested-by: NChris Wilson <chris@chris-wilson.co.uk>
Signed-off-by: NLionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: NChris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20171213171154.6201-1-lionel.g.landwerlin@intel.com

drivers/gpu/drm/i915/intel_device_info.c

@@ -416,7 +416,7 @@ static u32 read_timestamp_frequency(struct drm_i915_private *dev_priv)
 		return freq;
 	}
 
-	DRM_ERROR("Unknown gen, unable to compute command stream timestamp frequency\n");
+	MISSING_CASE("Unknown gen, unable to read command streamer timestamp frequency\n");
 	return 0;
 }