drm/i915: Linetime watermarks are a HSW feature

Linetime watermarks don't exist on ILK/SNB/IVB, so don't compute them
except on HSW.

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>