ALSA: x86: Fix sleep-in-atomic via i915 notification

i915 notification is executed in a spinlock, thus it must not sleep;
i.e. we can't use kmalloc with GFP_KERNEL or such.

For making it working properly, move the notification handler in a
work, and handle it gracefully.  We have already such a work, and it
was used just at the start.  This can be re-used in a more generic
hotplug handling.

Also, the patch adds the proper call of cancel_work_sync() to the
destructor.

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