A GitHub page for the Intel Media Driver for Video Acceleration API (VAAPI) has surfaced detailing the multimedia capabilities of Intel iGPUs including the Xe DG1 in Tiger Lake. From the video encode-decode support matrix, we get to see that Xe DG1 will support native 12-bit HEVC encode-decode along with decode support for 4:2:2 and 4:4:4 chroma sub-sampling, and VP9 12-bit and 12-bit 4:4:4 decode.
https://www.notebookcheck.net/Intel-Xe-DG1-in-Tiger-Lake-to-feature-native-12-bit-HEVC-and-VP9-support-more-laptops-with-HDR-displays-inbound.454322.0.html