drm/i915/dp: Use auxch precharge value of 5 everywhere

The default in the Sandybridge docs is 5, as on Ironlake, and I have no
reason to believe 3 would work any better.

Signed-off-by: Adam Jackson <ajax@redhat.com>
Acked-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>