drm/tegra: Propagate errors from drm_gem_plane_helper_prepare_fb()

Currently this function doesn't return an error, but that may change in
the future, so make sure to propagate any error codes that it might
return.
Signed-off-by: Thierry Reding <treding@nvidia.com>

drivers/gpu/drm/tegra/plane.c

@@ -200,11 +200,14 @@ int tegra_plane_prepare_fb(struct drm_plane *plane,
 			   struct drm_plane_state *state)
 {
 	struct tegra_dc *dc = to_tegra_dc(state->crtc);
+	int err;
 
 	if (!state->fb)
 		return 0;
 
-	drm_gem_plane_helper_prepare_fb(plane, state);
+	err = drm_gem_plane_helper_prepare_fb(plane, state);
+	if (err < 0)
+		return err;
 
 	return tegra_dc_pin(dc, to_tegra_plane_state(state));
 }