avcodec/h261dec: Initialize IDCT context during init

avcodec/h261dec: Initialize IDCT context during init

Before 998c9f15d1ca8c7489775ebcca51623b915988f1, initializing an
MpegEncContext's IDCT parts occured in ff_mpv_common_init() and this
has been called in h261_decode_frame(), not h261_decode_init().

Yet said commit factored this out of ff_mpv_common_init() and therefore
there is no reason any more not to set this during init as this commit
does.

Reviewed-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@outlook.com>