avformat/mxfenc: prefer to use the configured metadata

avformat/mxfenc: prefer to use the configured metadata

The metadata company_name, product_name, product_version from input
file will be deleted to avoid overwriting information
Please to test with below commands:
./ffmpeg -i ../fate-suite/mxf/Sony-00001.mxf -c:v copy -c:a copy out.mxf
and
./ffmpeg -i ../fate-suite/mxf/Sony-00001.mxf -c:v copy -c:a copy \
        -metadata company_name="xxx" \
        -metadata product_name="xxx" \
        -metadata product_version="xxx" \
        out.mxf

Reviewed-by: Tomas Härdin <tjoppen@acc.umu.se>
Signed-off-by: Limin Wang <lance.lmwang@gmail.com>