gdm-manager: Make sure the Plymouth client exist before we use it.

Hi @halfline : This is a minor fix, could you please take a glance to review it? The problem is the Plymouth default install its client under "/usr/bin", but gdm call it from the "/bin" directory. this action works under Fedora, but not work in other downstream distributions if the "/bin" and "/usr/bin" in 2 separate position. so I add a check. All your suggestions are welcome, thank you so much!