a64multienc: Do not entangle coded_frame

This change (and the following ones of the same kind) is mainly to
simplify wrapping this section with an #if FF_API block later on.

No functional changes are applied, the fields of the context coded_frame
fields are directly initialized, instead of keeping a reference to the
coded_frame itself.

Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
