drm/i915: Downgrade Gen9 Plane WM latency error

According to intel_read_wm_latency() it is perfectly legal for one WM
and all subsequent levels to be 0 (and the deeper powersaving states
disabled), so don't shout *ERROR*, over and over again.
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Ville Syrjala <ville.syrjala@linux.intel.com>
Acked-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180726161527.10516-1-chris@chris-wilson.co.uk

drivers/gpu/drm/i915/intel_pm.c

@@ -2942,8 +2942,8 @@ static void intel_print_wm_latency(struct drm_i915_private *dev_priv,
 		unsigned int latency = wm[level];
 
 		if (latency == 0) {
-			DRM_ERROR("%s WM%d latency not provided\n",
-				  name, level);
+			DRM_DEBUG_KMS("%s WM%d latency not provided\n",
+				      name, level);
 			continue;
 		}