pcm: dmix: Disable var_periodsize as default

The recently added variable period-size feature for dmix & co seems
causing a regression on some old applications; e.g. Audacious aborts
with an error:

  ALSA error: snd_pcm_hw_params_set_buffer_time_near failed: Invalid argument.

As a quick fix, disable the feature as default.  One can still enable
it via asoundrc.

Bugzilla: https://bugzilla.opensuse.org/show_bug.cgi?id=1033179


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