pcm: hw: add a helper function to issue appl_ptr without side-effects

pcm: hw: add a helper function to issue appl_ptr without side-effects

After starting, PCM substream shift its state to running and applications
can move appl_ptr by several ways. When status and control data of runtime
of the PCM substream is not mapped, the applications should issue appl_ptr
to kernel land. In this case, when any PCM frames is handled by mmap
operation, the applications should issue appl_ptr to update.

This commit adds a helper function for this purpose. To avoid unexpected
change of avail_min, this commit uses a flag just to update appl_ptr.

Signed-off-by: Takashi Sakamoto <o-takashi@sakamocchi.jp>
Signed-off-by: Takashi Iwai <tiwai@suse.de>