drm/etnaviv: restore ETNA_PREP_NOSYNC behaviour

This reverts commit cd34db4a (drm/etnaviv: Remove manual call to
reservation_object_test_signaled_rcu before wait), as the patch to turn
reservation_object_wait_timeout_rcu() into
reservation_object_test_signaled_rcu() with a 0 timeout has been reverted.
This causes the driver to call into the fence wait, even with a timeout of 0

The etnaviv BO cache depends on ETNA_PREP_NOSYNC to be wait-free, even if
the BO has attached fences, so restore the behaviour for this flag.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>