drm/etnaviv: remove unused function etnaviv_gem_mapping_reference

Hasn't been used for quite a while. There is no point in keeping
unused code around.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Guido Günther <agx@sigxcpu.org>