drm/rockchip: Check iommu itself instead of it's parent for device_is_available

When iommu itself is disabled in dts, we should
fallback to non-iommu buffer, check iommu parent
is meanless here.
Signed-off-by: Andy Yan <andy.yan@rock-chips.com>
Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Link: https://patchwork.freedesktop.org/patch/msgid/20210627084737.309163-1-andy.yan@rock-chips.com

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

@@ -374,7 +374,7 @@ static int rockchip_drm_platform_of_probe(struct device *dev)
 		}
 
 		iommu = of_parse_phandle(port->parent, "iommus", 0);
-		if (!iommu || !of_device_is_available(iommu->parent)) {
+		if (!iommu || !of_device_is_available(iommu)) {
 			DRM_DEV_DEBUG(dev,
 				      "no iommu attached for %pOF, using non-iommu buffers\n",
 				      port->parent);