drm/etnaviv: skip command stream validation on PPAS capable GPUs

With per-process address spaces in place, a rogue process submitting
bogus command streams can only hurt itself. There is no need to
validate the command stream before execution anymore.

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