ALSA: x86: Implement runtime PM

Although the driver has some PM callbacks, it doesn't do it right:
- the suspend callback doesn't handle to suspend the running PCM,
- the runtime PM ops are missing,
- pm_runtime_get_sync() isn't used at the right place.

This patch covers the above and provides the basic runtime PM
functionality.

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