omxvideodec: fix pool caps reference stealing
omxvideodec: fix pool caps reference stealing

gst_buffer_pool_config_get_params() doesn't ref the returning caps;
so gst_caps_replace() was unreffing the reference owned by the pool.

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

GStreamer/gst-omx,GStreamer/gst-omx,GStreamer/gst-omx