drm/i915: factor out modeset_update_power_wells

We'll need the same functionality for other HW generations. The support
for these will be added by upcoming patches.

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