When an amp in the activation path is associated with mixer controls,
activate_amp() tries to skip the initialization.  It's good, but only
if the mixer really initializes both mute and volume.  Otherwise,
either the mute of the volume is left uninitialized.

This patch adds this missing check and properly initialize the
partially controlled amps in an activation path.

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