ALSA: x86: Properly manage PCM substream lifetype

The PCM substream is referred not only in the PCM callbacks but also
in the irq handler and in the hotplug/unplug codes.  The latter code
paths don't take the PCM lock, thus the PCM may be released
unexpectedly while calling PCM helper functions or accessing
pcm->runtime fields.

This patch implements a simple refcount to assure the PCM substream
accessibility while the other codes are accessing.  It needed some
code refactoring in the relevant functions for avoiding the doubly
spinlocks.

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