drm/i915: Init important ns2501 registers

In my earlier rewrite I missed a few important registers. Thomas Richter
noticed that they're needed to make his machine resume correctly.

Looks like IEGD does a one time init of these three registers. We don't
have a good one time init place in the ns2501 driver, so let's just
stick them into the .mode_set() hook and see if that helps things along.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Thomas Richter <richter@rus.uni-stuttgart.de>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>