drm/i915: compute pipe_config earlier

To make decent modeset state checking possible (e.g. for the check
mode with atomic modesetting) we want to have the full pipe
configuration and state checks done before we touch the hw.

To ensure that all the little bits&pieces that are now moved to the
pipe_config handle this correctly, move its computation to the right
spot now, before we touch the hw in the disable_pipes step.

Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>