CMI8888 shows the stuttering playback when the snooping is disabled
on the audio buffer.  Meanwhile, we've got reports that CORB/RIRB
doesn't work in the snooped mode.  So, as a compromise, disable the
snoop only for CORB/RIRB and enable the snoop for the stream buffers.

The resultant patch became a bit ugly, unfortunately, but we still can
live with it.
Reported-and-tested-by: Geoffrey McRae <geoff@spacevs.com>
Cc: <stable@vger.kernel.org> # 3.17+
Signed-off-by: Takashi Iwai <tiwai@suse.de>