I followed HardwareVideoAcceleration - Debian Wiki and installed mesa-va-drivers but vainfo exits with error:
error: XDG_RUNTIME_DIR not set in the environment.
error: can't connect to X server!
libva info: VA-API version 1.10.0
libva error: vaGetDriverNameByIndex() failed with unknown libva error, driver_name = (null)
vaInitialize failed with error code -1 (unknown libva error),exit
Emby fails to detect hardware decoder and falls back to software transcode.