The recently introduced PCM info flag assures the call of ack ops at
each applptr change, and this is mandatory for the indirect PCM
helpers.

Also, with the proper ack callback, we need no longer prefill at
trigger start.  The relevant code can be killed.
Reviewed-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>