threads: always call thread_finish_setup for intra codecs

Intra codecs do not need an update_thread_context() function and never
call ff_thread_finish_setup(). They rely on ff_thread_get_buffer()
calling it. So call it even if the get_buffer2 function pointer is
avcodec_default_get_buffer2 and it has not been called before.
jpeg2kdec: output is native endian AV_PIX_FMT_XYZ12