drm/i915: Add debugs for the C8 vs. legacy LUT case

Leave a hint in dmesg when we reject a configuration attempting
to use C8 planes without the legacy LUT loaded.
Signed-off-by: NVille Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190513133904.20374-3-ville.syrjala@linux.intel.comReviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_color.c

@@ -959,8 +959,10 @@ static int check_luts(const struct intel_crtc_state *crtc_state)
 		return 0;
 
 	/* C8 relies on its palette being stored in the legacy LUT */
-	if (crtc_state->c8_planes)
+	if (crtc_state->c8_planes) {
+		DRM_DEBUG_KMS("C8 pixelformat requires the legacy LUT\n");
 		return -EINVAL;
+	}
 
 	degamma_length = INTEL_INFO(dev_priv)->color.degamma_lut_size;
 	gamma_length = INTEL_INFO(dev_priv)->color.gamma_lut_size;