avformat/movenc: Fix writing ACLR atoms for DNxHD

      Using the copy codec ACLR atoms where incorrectly written

During the creation of the ACLR atom we are assuming the vos_data
contains the DNxHD header. This change makes this explicit and
ensures we don't over write the stream with the extra_data.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>