drm/i915: Use intel_pipe_wm in hsw_find_best_results

Let's try to keep using the intermediate intel_pipe_wm representation
for as long as possible. It avoids subtle knowledge about the
internals of the hardware registers when trying to choose the
best watermark configuration.

While at it replace the memset() w/ zero initialization.

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>