drm/rockchip: fix NULL check on devm_kzalloc() return value

The right variable to check here is port, not dp.

This issue was detected using Coccinelle and the following semantic patch:

@@
expression x;
identifier fld;
@@

* x = devm_kzalloc(...);
  ... when != x == NULL
  x->fld
Signed-off-by: Gustavo A. R. Silva <garsilva@embeddedor.com>
Acked-by: Mark Yao <mark.yao@rock-chips.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170706215833.GA25411@embeddedgus

drivers/gpu/drm/rockchip/cdn-dp-core.c

@@ -1195,7 +1195,7 @@ static int cdn_dp_probe(struct platform_device *pdev)
 			continue;
 
 		port = devm_kzalloc(dev, sizeof(*port), GFP_KERNEL);
-		if (!dp)
+		if (!port)
 			return -ENOMEM;
 
 		port->extcon = extcon;