drm/i915: Move ILK/SNB/IVB over to the HSW WM code

The new HSW watermark code can now handle ILK/SNB/IVB as well, so
switch them over. Kill the old code.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Imre Deak <imre.deak@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>