util: move sending a pause command from flush_start to flush_stop
util: move sending a pause command from flush_start to flush_stop

REL OMXIL component requires a pause command to be sent before
a flush command according to the spec. The sequence of commands to
flush the component should be grouped into g_omx_core_flush_stop()
rather than sending separately.
When g_omx_core_flush_start() is invoked, g_omx_port_pause() is
sufficient to suspend decoding when an error occurs.

matsu/gst-openmax,matsu/gst-openmax