avcodec: get the decoder device before creating the VA

Only call decoder_GetDecoderDevice() in lavc_UpdateVideoFormat(). The
decoder_UpdateVideoOutput() is done afterwards.

After the call to lavc_UpdateVideoFormat() the received decoder device holds an
extra reference that needs to be released after the VA is created.

decoder_NewPicture() cannot be used until decoder_UpdateVideoOutput() is called.
So this patch temporarily breaks va modules relying on the picture_sys_t data
that used to be passed. It will be via the decoder device we just got.