Revert "backend/drm: populate cursor plane's current_fb"

This reverts commit 6c3d080e25.

Populating wlr_drm_plane.current_fb messes up the buffer's locking.
The previous buffer is released while it's still being displayed
on-screen.

backend/drm/drm.c

@@ -933,7 +933,7 @@ static bool drm_connector_set_cursor(struct wlr_output *output,
 			local_buf = wlr_buffer_lock(buffer);
 		}
 
-		bool ok = drm_fb_import(&plane->current_fb, drm, local_buf,
+		bool ok = drm_fb_import(&plane->pending_fb, drm, local_buf,
 			&plane->formats);
 		wlr_buffer_unlock(local_buf);
 		if (!ok) {