drm/i915: Use the active wm config for merging on ILK-BDW

ilk_program_watermarks() is supposed to merge the active watermarks from
all pipes. Thus we need to use the active config too instead of some
precomputed stuff.

Fixes: aa363136

 ("drm/i915: Calculate watermark configuration during atomic check (v2)")
Cc: Matt Roper <matthew.d.roper@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1452776015-22076-2-git-send-email-ville.syrjala@linux.intel.com