frigate: Pass DRI device for hardware acceleration

Enabling hardware acceleration using VA-API dramatically reduces
`ffmpeg` CPU usage.  For this to work, the Frigate container needs
access to the DRI device node.

frigate.container

@@ -16,6 +16,7 @@ Volume=/var/lib/frigate/media:/media/frigate:rw,z
 Volume=/var/lib/frigate/tmp:/tmp:rw,z
 Volume=/var/lib/frigate/config:/config:rw,z
 AddDevice=/dev/apex_0
+AddDevice=/dev/dri/renderD128
 Network=host
 
 [Service]