etnaviv: fill missing offset in etna_resource_get_handle

Without this gbm_bo_get_offset() can return 0 where it shouldn't.

Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Cc: <mesa-stable@lists.freedesktop.org>
(cherry picked from commit 1ccb8a07)