drm/i915: Refactor ilk_update_wm (v3)

Split ilk_update_wm() into two parts; one doing the programming
and the other the calculations.

v2: Fix typo in commit message

v3 (by Matt): Heavily rebased for current codebase.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>