drm/etnaviv: change return type of etnaviv_gem_obj_add to void

This function never fails, as it does nothing more than adding the GEM
object to the global device list. Making this explicit through the void
return type allows to drop some unnecessary error handling.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>