vga_switcheroo: Don't require handler init callback

This callback is a no-op in nouveau, and the upcoming apple-gmux
switcheroo support won't require it either. Rather than forcing drivers
to stub it out, just make it optional and remove the callback from
nouveau.

Signed-off-by: Seth Forshee <seth.forshee@canonical.com>
Signed-off-by: Matthew Garrett <mjg@redhat.com>

drivers/gpu/vga/vga_switcheroo.c

@@ -83,7 +83,8 @@ static void vga_switcheroo_enable(void)
 	struct vga_switcheroo_client *client;
 
 	/* call the handler to init */
-	vgasr_priv.handler->init();
+	if (vgasr_priv.handler->init)
+		vgasr_priv.handler->init();
 
 	list_for_each_entry(client, &vgasr_priv.clients, list) {
 		if (client->id != -1)