This patch moves the mixer creation code in Cirrus codec driver from
its own build_controls callback to snd_hda_gen_add_kctl() for
simplification.

As a bonus, this allows us to remove the cs421x_build_controls as it
becomes identical with snd_hda_gen_build_controls().
Signed-off-by: Takashi Iwai <tiwai@suse.de>