ALSA: hda: Initialize ext-bus-specific fields in snd_hdac_bus_init(), too

Some fields in snd_hdac_bus are ext-bus specific, but they still
should be initialized in snd_hdac_bus_init() for consistency, at
least, for the ones that do need the explicit initialization like the
list head.

Also move the lock field to the more appropriate place and correct the
comment to reflect the recent change where it serves for both the
display power and the link management.

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