drm/i915: Use the ilk_disable_lp_wm() return value

ilk_disable_lp_wm() will tell us whether the LP1+ watermarks were
disabled or not, and hence whether we need to for the vblank wait or
not. Let's use that information to eliminate some useless vblank
waits.

Signed-off-by: Ville Syrjälä <ville.syrjala@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1480354637-14209-3-git-send-email-ville.syrjala@linux.intel.com
Reviewed-by: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>

drivers/gpu/drm/i915/intel_display.c

@@ -5114,10 +5114,8 @@ static void intel_pre_plane_update(struct intel_crtc_state *old_crtc_state)
 	 *
 	 * WaCxSRDisabledForSpriteScaling:ivb
 	 */
-	if (pipe_config->disable_lp_wm) {
-		ilk_disable_lp_wm(dev);
+	if (pipe_config->disable_lp_wm && ilk_disable_lp_wm(dev))
 		intel_wait_for_vblank(dev_priv, crtc->pipe);
-	}
 
 	/*
 	 * If we're doing a modeset, we're done.  No need to do any pre-vblank