When we have a loopback mixer control, this should manage the state
whether the output paths include the aamix or not.  But the current
code blindly initializes the output paths with aamix = true, thus the
aamix is enabled unless the loopback mixer control is changed.

Also, update_aamix_paths() called by the loopback mixer control put
callback invokes snd_hda_activate_path() with aamix = true even for
disabling the mixing.  This leaves the aamix path even though the
loopback control is turned off.

This patch fixes these issues:
- Introduced aamix_default() helper to indicate whether with_aamix is
  true or false as default
- Fix the argument in update_aamix_paths() for disabling loopback

Reported-by: Lydia Wang <LydiaWang@viatech.com.cn>
Cc: <stable@vger.kernel.org> [v3.9+]
Signed-off-by: Takashi Iwai <tiwai@suse.de>