drm/etnaviv: move job context pointer to etnaviv_gem_submit

The context isn't really related to the cmdbuf, but is a property of
the job. This has been missed when moving to a properly refcounted
etnaviv_gem_submit.

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