drm/i915: g4x/vlv: fix dp aux interrupt mask

Fix typo possibly leading to timed out DP aux transactions on ports C,D.

Introduced in:

Commmit 4aeebd74
Author: Daniel Vetter <daniel.vetter@ffwll.ch>
Date:   Thu Oct 31 09:53:36 2013 +0100

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=72210


Signed off-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>