omxvideodec: don't import OMX buffers from downstream

omxvideodec: don't import OMX buffers from downstream

We already have code configuring the encoder stride and slice height
when receiving the first buffer from upstream.
We don't have an equivalent when the encoder is exporting its buffers to the
decoder.

There is no point adding it and making the code even more
complex as we wouldn't gain anything by exporting from the encoder to
the decoder. The dynamic buffer mode already ensures 0-copy between OMX
components.

https://bugzilla.gnome.org/show_bug.cgi?id=796918