avcodec/mpegvideo: Factor ff_mpv_decode_init() out

Reviewed-by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avcodec/mpegvideo: set codec tags in ff_mpv_decode_init()

Fixes Ticket3912

Reviewed-by: Benoit Fouet <benoit.fouet@free.fr>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>