Diploma isn't a degree course. So in most cases, if you have done 10+2, then you can apply for the Police post with the eligibility of 10+2 pass. Apart from this, you can't apply for the police post which have the eligibility for Graduation. Diploma isn't considered as a graduation course.

For the police service examination in India you can consider visiting the link below:

https://competition.careers360.com/defence-exams/police-exams

I hope this helps. Wish you a great eve!