It could also be a gstreamer-vaapi bug since it happens only since gstreamer
1.14, but I have seen nobody else complain anywhere, so it must work for some
people at least.

RX 480, recent mesa git revisions, Linux 4.15 - drm-next-4.18-wip

To test:
gst-launch-1.0 videotestsrc ! vaapih264enc ! "video/x-h264,profile=baseline" !
h264parse ! matroskamux ! filesink location=output.mkv

With gstreamer 1.14 this never starts encoding and produces a 0 byte file when
cancelling with ctrl+c.

With gstreamer 1.12.4 it worked fine.