drm/i915: Move LP1+ watermark merging out from hsw_compute_wm_results()

I want to convert hsw_find_best_result() to use intel_pipe_wm, so we
need to move the merging to happen outside hsw_compute_wm_results().

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>