drm/i915: disable cpt phase pointer fdi rx workaround

We've originally added this in

commit 291427f5
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Fri Jul 29 12:42:37 2011 -0700

    drm/i915: apply phase pointer override on SNB+ too

and then copy-pasted it over to ivb/ppt. The w/a was originally added
for ilk/ibx in

commit 5b2adf89
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Thu Oct 7 16:01:15 2010 -0700

    drm/i915: add Ironlake clock gating workaround for FDI link training

and fixed up a bit in

commit 6f06ce18
Author: Jesse Barnes <jbarnes@virtuousgeek.org>
Date:   Tue Jan 4 15:09:38 2011 -0800

    drm/i915: set phase sync pointer override enable before setting phase sync pointer

It turns out that this w/a isn't actually required on cpt/ppt and
positively harmful on ivb/ppt when using fdi B/C links - it results in
a black screen occasionally, with seemingfully everything working as
it should. The only failure indication I've found in the hw is that
eventually (but not right after the modeset completes) a pipe underrun
is signalled.

Big thanks to Arthur Runyan for all the ideas for registers to check
and changes to test, otherwise I couldn't ever have tracked this down!

Cc: "Runyan, Arthur J" <arthur.j.runyan@intel.com>
Cc: stable@vger.kernel.org
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>