Added a new helper function snd_hda_power_sync() to trigger the
power-saving manually.  It's an inline function call to
snd_hda_power_save() helper function.

Together with this addition, snd_hda_power_up*() and
snd_hda_power_down() functions are inlined to a call of the same
snd_hda_power_save() helper function.
Signed-off-by: Takashi Iwai <tiwai@suse.de>