drm/i915: s/haswell_update_wm/ilk_update_wm/

We use this hook starting from ILK onwards, so change the prefix
accordingly. Also rename functions/struct names used from
haswell_update_wm that are relevant to ILK already.

No functional change.

Signed-off-by: Imre Deak <imre.deak@intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>