drm/omap: fix possible object reference leak

The call to of_find_matching_node returns a node pointer with refcount
incremented thus it must be explicitly decremented after the last
usage.

Detected by coccinelle with the following warnings:
drivers/gpu/drm/omapdrm/dss/omapdss-boot-init.c:212:2-8: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 209, but without a corresponding object release within this function.
drivers/gpu/drm/omapdrm/dss/omapdss-boot-init.c:237:1-7: ERROR: missing of_node_put; acquired a node pointer with refcount incremented on line 209, but without a corresponding object release within this function.
Signed-off-by: Wen Yang <wen.yang99@zte.com.cn>
Reviewed-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Reviewed-by: Mukesh Ojha <mojha@codeaurora.org>
Cc: Tomi Valkeinen <tomi.valkeinen@ti.com>
Cc: David Airlie <airlied@linux.ie>
Cc: Daniel Vetter <daniel@ffwll.ch>
Cc: Sebastian Reichel <sebastian.reichel@collabora.com>
Cc: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-kernel@vger.kernel.org
Cc: Markus Elfring <Markus.Elfring@web.de>
Signed-off-by: Tomi Valkeinen <tomi.valkeinen@ti.com>
Link: https://patchwork.freedesktop.org/patch/msgid/1554692313-28882-2-git-send-email-wen.yang99@zte.com.cn

drivers/gpu/drm/omapdrm/dss/omapdss-boot-init.c

@@ -192,7 +192,7 @@ static int __init omapdss_boot_init(void)
 	dss = of_find_matching_node(NULL, omapdss_of_match);
 
 	if (dss == NULL || !of_device_is_available(dss))
-		return 0;
+		goto put_node;
 
 	omapdss_walk_device(dss, true);
 
@@ -217,6 +217,8 @@ static int __init omapdss_boot_init(void)
 		kfree(n);
 	}
 
+put_node:
+	of_node_put(dss);
 	return 0;
 }