drm/tegra: hub: Do not enable orphaned window group

Though the unconditional enable/disable code is not a final solution,
we don't want to run into a NULL pointer situation when window group
doesn't link to its DC parent if the DC is disabled in Device Tree.

So this patch simply adds a check to make sure that window group has
a valid parent before running into tegra_windowgroup_enable/disable.

Signed-off-by: Nicolin Chen <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>

drivers/gpu/drm/tegra/hub.c

···
 	for (i = 0; i < hub->soc->num_wgrps; i++) {
 		struct tegra_windowgroup *wgrp = &hub->wgrps[i];
 
-		tegra_windowgroup_enable(wgrp);
+		/* Skip orphaned window group whose parent DC is disabled */
+		if (wgrp->parent)
+			tegra_windowgroup_enable(wgrp);
 	}
 
 	return 0;
···
 	for (i = 0; i < hub->soc->num_wgrps; i++) {
 		struct tegra_windowgroup *wgrp = &hub->wgrps[i];
 
-		tegra_windowgroup_disable(wgrp);
+		/* Skip orphaned window group whose parent DC is disabled */
+		if (wgrp->parent)
+			tegra_windowgroup_disable(wgrp);
 	}
 }