drm/i915: Start WM computation from scratch on ILK-BDW

ilk_compute_pipe_wm() assumes as zeroed pipe_wm structure when it
starts. We used to pass such a zeroed struct in, but this got broken
when the pipe_wm structure got embedded in the crtc state.

To fix it without too much fuzz, we need to resort to a memset().

Fixes: 4e0963c7

 ("drm/i915: Calculate pipe watermarks into CRTC state (v3)")
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/1452776015-22076-1-git-send-email-ville.syrjala@linux.intel.com