pcm: Avoid lock for snd_pcm_nonblock()

snd_pcm_nonblock() is called as snd_pcm_abort().  Since
snd_pcm_abort() is called often from a signal handler to clean things
up (e.g. aplay does it), we may face a deadlock if the signal is
raised during the locked operation.

There can be some way to check the deadlock state, but they would cost
much.  Since the race condition of snd_pcm_nonblock() is quite small,
let's just drop the locking inside snd_pcm_nonblock() as a
workaround.

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