idct: remove call to ff_idctdsp_init from ff_MPV_common_init

One step in untangling the mpegvideo code and fixing some problems in
the order that initialization is being done in h263dec and h261dec.
h263dec: Fix order of initialization

ff_MPV_common_init requires the frame dimensions which get parsed in
*_decode_picture_header.
h261dec: Fix order of initialization

ff_MPV_common_init requires the frame dimensions which get parsed in
h261_decode_picture_header.