drm/i915/skl: Store the new WM state at the very end of the update

We're going to add a new step, let's not hide the copy of the new WM
state inside one inner function, but as a 1st level operation in the WM
update.

v2: Split the staging results structure to not exceed the 1Kb stack
    allocation in skl_update_wm()

Reviewed-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>