I just found to out today that if you have SQL Server 2017 Enterprise with
Software Assurance, then you also get an On Prem Power BI license. So no
need to purchase the Premium or Power BI Pro license.
I'm unsure of the exact release date for SQL Server 2017, but I imagine
it'll probably arrive not long after the June 1st release of Power BI on
Prem. However you can download the preview here.
Shoot me an email if you have any questions or if I can help with any of
your Power BI needs.