drm/i915: don't save/restore DP regs for kms

We completely compute these anew in each modeset, hence we don't rely
on them containing anything valid after resume.

To avoid breaking any ums setup due to reordering of the reads/writes
simply don't reorder anything, but bracket the reads/writes into if
(!kms) conditionals. More churn, but safer.

v2: Fixup the logic, noticed by Paulo Zanoni.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Tested-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>