h264: fix bit depth changes with frame threading

AVCodecContext.bits_per_raw_sample is updated from the previous thread
in the generic update function before the codec specific update_thread
function is called. The check for reinitialization of dsp functions uses
bits_per_raw_sample. When called from update_thread_context it will be
already at the current value and the dsp functions aren't updated if
only the bit depth changes.
