$ export | grep -e XAUTH -e DISP
declare -x DISPLAY=":0"
declare -x XAUTHORITY="/home/wtt/.Xauthority"

$ vainfo
libva info: VA-API version 0.34.0
libva info: va_getDriverName() returns 0
libva info: Trying to open /wld/install/lib/dri/i965_drv_video.so
libva info: Found init function __vaDriverInit_0_34
libva info: va_openDriver() returns 0
vainfo: VA-API version: 0.34 (libva 1.2.1)
vainfo: Driver version: Intel i965 driver - 1.2.0
vainfo: Supported profile and entrypoints
      VAProfileNone                   : VAEntrypointVideoProc
      VAProfileMPEG2Simple            : VAEntrypointVLD
      VAProfileMPEG2Simple            : VAEntrypointEncSlice
      VAProfileMPEG2Main              : VAEntrypointVLD
      VAProfileMPEG2Main              : VAEntrypointEncSlice
      VAProfileH264Baseline           : VAEntrypointVLD
      VAProfileH264Baseline           : VAEntrypointEncSlice
      VAProfileH264Main               : VAEntrypointVLD
      VAProfileH264Main               : VAEntrypointEncSlice
      VAProfileH264High               : VAEntrypointVLD
      VAProfileH264High               : VAEntrypointEncSlice
      VAProfileVC1Simple              : VAEntrypointVLD
      VAProfileVC1Main                : VAEntrypointVLD
      VAProfileVC1Advanced            : VAEntrypointVLD

$ GST_DEBUG=vaapi:5,vaapidecode:5,vaapisink:5 gst-launch-1.0 filesrc location=/home/wtt/Videos/Sintel.2010.1080p.mp4 ! qtdemux ! vaapidecode ! vaapisink
0:00:00.062270306  1182      0x14414f0 DEBUG            vaapidecode gstvaapidecode.c:841:gst_vaapidecode_query: sharing display (nil)
0:00:00.062458300  1182      0x14414f0 INFO                   vaapi gstvaapidisplay.c:112:libgstvaapi_init_once: gstreamer-vaapi version 0.5.5
0:00:00.062630877  1182      0x14414f0 ERROR            vaapidecode gstvaapidecode.c:807:gst_vaapidecode_ensure_allowed_caps: failed to retrieve VA display
0:00:00.062760425  1182      0x14414f0 DEBUG            vaapidecode gstvaapidecode.c:841:gst_vaapidecode_query: sharing display (nil)
0:00:00.062989869  1182      0x14414f0 DEBUG            vaapidecode gstvaapidecode.c:841:gst_vaapidecode_query: sharing display (nil)
Setting pipeline to PAUSED ...
0:00:00.063362930  1182      0x14414f0 DEBUG            vaapidecode gstvaapidecode.c:841:gst_vaapidecode_query: sharing display (nil)
ERROR: Pipeline doesn't want to pause.
Setting pipeline to NULL ...
Freeing pipeline ...

$ echo $?
255