I think this phrase is polite, but you would say 'Thank you in advance for your help.' It means you are thanking the person before they give the help.
As the previous answer pointed out, the phrase can be taken as impolite if you are assuming incorrectly that someone wants to help you. However, it is commonly used politely.
When you say 'thank you for your help in advance' it means that they helped you before you asked, or that they are helping you before something else, like a test for example.
Also, you asked, is it politely or impolitely. This is the incorrect form. You would say, "Is it polite or impolite." No -ly at the end of the word.
