avcodec/mpegvideo: Merge ff_mpv_decode_defaults into ff_mpv_decode_init

avcodec/mpegvideo: Merge ff_mpv_decode_defaults into ff_mpv_decode_init

These two are always called directly after each other (with the
exception of the calls in mpeg_decode_init() where some irrelevant
modifications of the avctx (which could just as well be done before
ff_mpv_decode_defaults(), because it doesn't have a pointer to the
AVCodecContext at all and therefore can't see these modifications at
all) are performed in between), so merge ff_mpv_decode_defaults() in
ff_mpv_decode_init().

Reviewed-by: Anton Khirnov <anton@khirnov.net>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>