more efficiently determine style for instance marker

Only retrieve the associated instance if necessary.

Change-Id: Ie64ce5f6ba7d161a4e7cce7f419d2cdba4048eaa
Reviewed-on: http://gerrit.esdi-community.eu/3106
Tested-by: Jenkins <sitemple@igd.fraunhofer.de>
Reviewed-by: Simon Templer <simon.templer@igd.fraunhofer.de>