drm/i915: Use struct intel_crtc in legacy platform wm code

Unify our approach to things by using intel_crtc instead of drm_crtc.
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1477946245-14134-6-git-send-email-ville.syrjala@linux.intel.comReviewed-by: Chris Wilson <chris@chris-wilson.co.uk>