pcm: Skip avail_min check during draining

snd_pcm_wait() & co checks the current avail value and returns
immediately if it satisfies <= avail_min condition.  It's good in
general except for one situation: draining.  When the draining is
being performed in the non-blocking mode, apps are supposed to wait
via poll(), typically via snd_pcm_wait().  So this ends up with the
busy loop because of the immediate return from snd_pcm_wait().

A simple workaround is to put the PCM state check and ignore the
avail_min condition if it's DRAINING state.  The equivalent check is
found in the kernel xfer code, too.

Signed-off-by: Takashi Iwai <tiwai@suse.de>