Hello,
It would be nice to read, what is supported in omx-gstreamer for dm8148.
I do not see in your code nothing about the pads.
Please refer to http://gstreamer.freedesktop.org/, there is a documentation regarding gstreamer including example how to build our own application(http://gstreamer.freedesktop.org/data/doc/gstreamer/head/manual/html/chapter-helloworld.html).
Let me know is this working for you.
BR
Margarita