drm/i915: Let's hope future platforms will use the same WM code as SKL

Given the history, there's some chance we'll keep the same WM code for a
bit (previously, we were able to reuse the same WM code from ILK to BDW,
so that sounds like a fair assumption).

Signed-off-by: Damien Lespiau <damien.lespiau@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>