drm/etnaviv: submit support for in-fences

Loosely based on commit f0a42bb5 ("drm/msm: submit support for
in-fences"). Unfortunately, struct drm_etnaviv_gem_submit doesn't have
a flags field yet, so we have to extend the structure and trust that
drm_ioctl will clear the flags for us if an older userspace only submits
part of the struct.

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Gustavo Padovan <gustavo.padovan@collabora.com>
Reviewed-by: Sumit Semwal <sumit.semwal@linaro.org>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>