h264_metadata: Localize code for display orientation

h264_metadata: Localize code for display orientation

The recent changes to h264_metadata (enabled by the recent changes to
ff_cbs_write_packet) made it possible to add side_data to the output
packet at any place, not only after the output packet has been written
and the properties of the input packet copied. This means that one can
now localize the code to add display orientation side-data to the packet
to the place dealing with said display-orientation.

Furthermore, the documentation of av_display_rotation_set states that
the matrix will be fully overwritten by it, so there is no need to
allocate it with av_mallocz.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>