drm/i915: Preserve the DDI link reversal configuration

Similarly to:

  commit 6a0d1df3d3a0d2370541164eb0595fe35dcd6de3
  Author: Damien Lespiau <damien.lespiau@intel.com>
  Date:   Tue Dec 11 15:18:28 2012 +0000

      drm/i915: Preserve the FDI line reversal override bit on CPT

DDI port support lane reversal to easy the PCB layouting work. Let's
preserve the bit configured by the BIOS (until we find how to correctly
retrieve the information from the VBT, but this does sound more fragile
then just relying on the BIOS that has, hopefully, been validated
already.

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Cc: stable@vger.kernel.org
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>