drm/etnaviv: expedited MMU fault handling

The GPU is halted when it hits a MMU exception, so there is no point in
waiting for the job timeout to expire or try to work out if the GPU is
still making progress in the timeout handler, as we know that the GPU
won't make any more progress.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>