drm/i915: Fix watermark code for BDW

Looks like I forgot to update the ILK/SNB/IVB watermark patches to deal
with BDW. Add the relevant BDW checks to make sure we take the HSW
codepaths on BDW as well.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Reviewed-by: Paulo Zanoni <paulo.r.zanoni@intel.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>