drm/etnaviv: remove -EAGAIN handling from submit path

Now that the userptr BO handling doesn't rely on the userspace restarting
the submit after object population, there is no need to special case the
-EAGAIN return value anymore.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>