drm/i915: move border color writes to pfit_enable

Writing hw registers from compute_config?
Just say no!

In this case not too horrible since we write a constant 0, and only
debugging would put something else in there. But while checking that
code I've noticed that this register disappeared on pch platforms, so
fix that up, too.

And adjust the comment a bit, it's outdated.

Reviewed-by: Mika Kuoppala <mika.kuoppala@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>