drm/i915: Don't merge LP1+ watermarks on ILK/SNB/IVB when multiple pipes are enabled

Multi-pipe LP1+ watermarks are a HSW+ feature, so let's not do it on
earlier generations.

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>