ALSA: hda - Split snd_hda_build_pcms()

snd_hda_build_pcms() does actually three things: let the codec driver
build up hda_pcm list, set the PCM default values, and call the
attach_pcm bus ops for each hda_pcm instance.  The former two are
basically independent from the bus implementation, so it'd make the
code a bit more readable.

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