drm/i915: move modeset_update_power_wells earlier

These functions will be needed by the valleyview specific power well
update functionality added in an upcoming patch, so move them earlier.

No functional change.

v2:
- no change
v3:
- rebase on latest -nightly

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Jesse Barnes <jbarnes@virtuousgeek.org> (v2)
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>