dca: Move ff_dca_convert_bitstream() to the DCA common code

This makes the DCA parser and decoder independent.
cosmetics: Remove unnecessary extern keywords from function declarations
mpeg12: Move some ff_mpeg1_* function declarations to a more suitable place
Split MPEG-1/2 decoder code off from MPEG-1/2 common code
mpeg12: Move Mpeg1Context declaration to the only place it is used