ALSA: HDA: Use LPIB position fix for Macbook Pro 7,1

Several users have reported "choppy" audio under the 3.2 kernel,
and that changing position_fix to 1 has resolved their problem.
The chip is an nVidia Corporation MCP89 High Definition Audio,
[10de:0d94] (rev a2).

Cc: stable@kernel.org (v3.2+)
BugLink: https://bugs.launchpad.net/bugs/909419


Signed-off-by: David Henningsson <david.henningsson@canonical.com>
Signed-off-by: Takashi Iwai <tiwai@suse.de>