drm: Only take cursor locks when the cursor plane exists

I thought I've fixed this, but maybe not. Anyway, clearly broken, and
easy fix.

Cc: Tony Lindgren <tony@atomide.com>
Reported-by: Tony Lindgren <tony@atomide.com>
Fixes: b95ff0319a82 ("drm: Remove drm_modeset_(un)lock_crtc")
Cc: Harry Wentland <harry.wentland@amd.com>
Cc: Maarten Lankhorst <maarten.lankhorst@linux.intel.com>
Cc: Daniel Vetter <daniel.vetter@intel.com>
Cc: Jani Nikula <jani.nikula@linux.intel.com>
Cc: Sean Paul <seanpaul@chromium.org>
Cc: David Airlie <airlied@linux.ie>
Cc: dri-devel@lists.freedesktop.org
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Tested-by: Tony Lindgren <tony@atomide.com>
Reviewed-by: Daniel Stone <daniels@collabora.com>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/20170407164817.28272-1-daniel.vetter@ffwll.ch

drivers/gpu/drm/drm_plane.c

@@ -720,15 +720,15 @@ retry:
 	ret = drm_modeset_lock(&crtc->mutex, &ctx);
 	if (ret)
 		goto out;
-	ret = drm_modeset_lock(&crtc->cursor->mutex, &ctx);
-	if (ret)
-		goto out;
-
 	/*
 	 * If this crtc has a universal cursor plane, call that plane's update
 	 * handler rather than using legacy cursor handlers.
 	 */
 	if (crtc->cursor) {
+		ret = drm_modeset_lock(&crtc->cursor->mutex, &ctx);
+		if (ret)
+			goto out;
+
 		ret = drm_mode_cursor_universal(crtc, req, file_priv, &ctx);
 		goto out;
 	}