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: NVille Syrjälä <ville.syrjala@linux.intel.com>
Signed-off-by: NMatt Roper <matthew.d.roper@intel.com>
Reviewed-by: NMaarten Lankhorst <maarten.lankhorst@linux.intel.com>
Signed-off-by: NDaniel Vetter <daniel.vetter@ffwll.ch>