Once we push the job, the scheduler could run it and free it.  So, if
we want to reference their fences, we need to grab them before then.
I haven't seen this happen in many days of conformance test runtime,
but let's still close the race.
Signed-off-by: Eric Anholt <eric@anholt.net>
Fixes: 57692c94 ("drm/v3d: Introduce a new DRM driver for Broadcom V3D V3.x+")
Link: https://patchwork.freedesktop.org/patch/254119/Reviewed-by: Boris Brezillon <boris.brezillon@bootlin.com>