drm/i915: Fix FB WM for HSW

Due to a misplaced memset(), we never actually enabled the FBC WM on HSW.
Move the memset() to happen a bit earlier, so that it won't clobber
results->enable_fbc_wm.

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>