Just in case someone reading this misunderstands. This discussion is limited to the permissability of asking the Prophet at his graveside to make dua to Allah for us. Asking the Prophet directly for help would be shirk. Prophet Eesa alayhi salaam was raised up alive and hasnt passed away yet. So this question doesnt even arise. Regarding the other Prophets, I think those who believe in this practice allow it for them as well provided the request is only made at the graveside.