drm/i915: Remove dead update_wm_pre assignment from SKL wm code

SKL+ do not use crtc_state->update_wm_pre, so there is absolutely no
point it setting it. crtc_state->update_wm_pre only exists as a
temporary hack for pre-g4x platforms until we redo their
watermarks to be be atomic.

Cc: Stanislav Lisovskiy <stanislav.lisovskiy@intel.com>
Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181113172330.26069-3-ville.syrjala@linux.intel.comReviewed-by: Rodrigo Vivi <rodrigo.vivi@intel.com>

drivers/gpu/drm/i915/intel_pm.c

@@ -5568,12 +5568,6 @@ skl_compute_wm(struct drm_atomic_state *state)
 
 		if (changed)
 			results->dirty_pipes |= drm_crtc_mask(crtc);
-
-		if ((results->dirty_pipes & drm_crtc_mask(crtc)) == 0)
-			/* This pipe's WM's did not change */
-			continue;
-
-		intel_cstate->update_wm_pre = true;
 	}
 
 	skl_print_wm_changes(intel_state);