drm/i915: Don't clobber crtc->new_config when nothing changes

When doing a nop modeset we currently leave crtc->new_config point at
the already freed temporary pipe_config. That will anger the sanity
checks in intel_modeset_update_state() when the nop modeset gets
followed by a GPU reset on gen3/4 where the display block gets fully
reinitialized during the reset.

So leave crtc->new_config alone until we know a modeset is actually
required.

Cc: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>