gstreamer-vaapi 0.5.8 がリリースされたので、
早速インストールしてみたのだが、gst-launchで想定通り動かないでござるの巻
vaapisink だけは動くことを確認できたのだけど、下のコマンドはNG

$ gst-launch-1.0 filesrc location=./100000.ts ! decodebin ! videoconvert ! vaapiencode_h264 ! avimux ! filesink location=test.avi

$ vainfo
libva info: VA-API version 0.34.0
libva info: va_getDriverName() returns 0
libva info: Trying to open /usr/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 - 1.2.2
vainfo: Supported profile and entrypoints
VAProfileNone : VAEntrypointVideoProc
VAProfileMPEG2Simple : VAEntrypointVLD
VAProfileMPEG2Simple : VAEntrypointEncSlice
VAProfileMPEG2Main : VAEntrypointVLD
VAProfileMPEG2Main : VAEntrypointEncSlice
VAProfileH264Main : VAEntrypointVLD
VAProfileH264Main : VAEntrypointEncSlice
VAProfileH264High : VAEntrypointVLD
VAProfileH264High : VAEntrypointEncSlice
VAProfileVC1Simple : VAEntrypointVLD
VAProfileVC1Main : VAEntrypointVLD
VAProfileVC1Advanced : VAEntrypointVLD
VAProfileJPEGBaseline : VAEntrypointVLD
VAProfileH264ConstrainedBaseline: VAEntrypointVLD
VAProfileH264ConstrainedBaseline: VAEntrypointEncSlice