drm/i915: debug log for REPLY_ACK missing

Adding a debug log when the DP_AUX_NATIVE_REPLY_ACK is missing
for aksv write. This helps to locate the possible non responding
DP HDCP sinks.

v2:
  Rewritten for readability [Sean Paul]
Signed-off-by: Ramalingam C <ramalingam.c@intel.com>
Reviewed-by: Sean Paul <sean@poorly.run>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/1544010283-20223-4-git-send-email-ramalingam.c@intel.com

drivers/gpu/drm/i915/intel_dp.c

@@ -5616,7 +5616,12 @@ int intel_dp_hdcp_write_an_aksv(struct intel_digital_port *intel_dig_port,
 	}
 
 	reply = (rxbuf[0] >> 4) & DP_AUX_NATIVE_REPLY_MASK;
-	return reply == DP_AUX_NATIVE_REPLY_ACK ? 0 : -EIO;
+	if (reply != DP_AUX_NATIVE_REPLY_ACK) {
+		DRM_DEBUG_KMS("Aksv write: no DP_AUX_NATIVE_REPLY_ACK %x\n",
+			      reply);
+		return -EIO;
+	}
+	return 0;
 }
 
 static int intel_dp_hdcp_read_bksv(struct intel_digital_port *intel_dig_port,