This might be the answer:
Newer AMD drivers, including all of those that are compatible with the R9 380x, now no longer support OpenCL. (The hardware for OpenCL support is there; it's just that all recent drivers have that support disabled in the driver code.) Thus, you now effectively have no GPGPU support at all with any 300- or 400-series AMD GPUs. A rollback to an older driver may help - but then you'd have to downgrade to an older AMD GPU for that.
i find it difficult to believe amd would do such a thing, do you have any links to articles etc to show this?
there have been several problems/bugs with premiere and amd and intel cards, but your amd card should work. if you have an cpu with internal graphics you might want to try disabling that and see if premiere can then find and use the amd r9 380x.