dnn_backend_openvino.c: separate function execute_model_ov

function fill_model_input_ov and infer_completion_callback are
extracted, it will help the async execution for reuse.

Signed-off-by: Xie, Lin <lin.xie@intel.com>
Signed-off-by: Wu Zhiwen <zhiwen.wu@intel.com>
Signed-off-by: Guo, Yejun <yejun.guo@intel.com>
