Install libomxil-bellagio0 and make sure mesa has been built with
--enable-omx. Check there is /usr/lib/libomxil-bellagio0/libomx_mesa.so
then run omxregister-bellagio to regenerate $XDG_DATA_HOME/.omxregister
Then omxh264dec and omxmpeg2dec should load.
https://bugzilla.gnome.org/show_bug.cgi?id=782926
in-port-index=0
out-port-index=1
hacks=event-port-settings-changed-ndata-parameter-swap;no-component-role;no-disable-outport;drain-may-not-return
+
+[omxh264dec]
+type-name=GstOMXH264Dec
+core-name=/usr/lib/libomxil-bellagio.so.0
+component-name=OMX.mesa.video_decoder.avc
+rank=0
+in-port-index=0
+out-port-index=1
+
+[omxmpeg2dec]
+type-name=GstOMXMPEG2VideoDec
+core-name=/usr/lib/libomxil-bellagio.so.0
+component-name=OMX.mesa.video_decoder.mpeg2
+rank=0
+in-port-index=0
+out-port-index=1