drm/tegra: Fix return value

Return NULL instead of 0 in host1x_bo_lookup().

Signed-off-by: Thierry Reding <thierry.reding@gmail.com>
diff --git a/drivers/gpu/host1x/drm/gr2d.c b/drivers/gpu/host1x/drm/gr2d.c
index aca72fc..7efd97b 100644
--- a/drivers/gpu/host1x/drm/gr2d.c
+++ b/drivers/gpu/host1x/drm/gr2d.c
@@ -84,7 +84,7 @@
 
 	gem = drm_gem_object_lookup(drm, file, handle);
 	if (!gem)
-		return 0;
+		return NULL;
 
 	mutex_lock(&drm->struct_mutex);
 	drm_gem_object_unreference(gem);