drm/i915: Avoid computing invalid WM levels when sprites/scaling is enabled

On ILK/SNB only LP0/1 watermarks can be enabled when sprites are
enabled, and on ILK/SNB/IVB sprite scaling is limited to LP0 only.

So we can avoid computing the extra levels we're never going to use.

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>