ALSA: hda/conexant - Clean up beep code

Like the previous commit for Realtek codec, the similar cleanup work
can be applied to Conexant codec, too.  A slight difference is that
the call of cx_auto_parse_beep() is moved after
snd_hda_gen_parse_auto_config().  It's not strictly needed, but it'd
be good to make the creation of such beep mixers at the end, which
matches with the former situation.

Along with this conversion, cx_auto_build_controls() becomes just
calling snd_hda_gen_build_controls(), so it's simply replaced with
snd_hda_gen_build_controls.

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