drm/i915: Remove some useless zeroing on skl+ wm calculations

drm/i915: Remove some useless zeroing on skl+ wm calculations

We memset(0) the entire watermark struct the start, so there's no
need to clear things later on.

v2: Rebase due to some stale w/a removal

Reviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181114210729.16185-7-ville.syrjala@linux.intel.com
Reviewed-by: Matt Roper <matthew.d.roper@intel.com>
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>