There are two issues with HEVC_MAIN_10. First, you’re right that the driver doesn’t support it yet. For future reference, that’s tracked in internal bug 1617735.
The other issue is an API one: VDPAU doesn’t provide a way to allocate, render to, or display a 10-bit surface. So even if bug 1617735 is fixed, the driver would have to dither down to 8 bits for display. Internal bug 1632828 tracks adding 10-bit surface support to the API and the driver.
Sorry I don’t have better news with regard to enabling these features.