drm/etnaviv: fix workaround for GC500

The hardware description macros define the mask and shifts the wrong
way around for the intended use, leading to the condition never being
true and the chip revision ending up with the wrong value.

Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Acked-by: Christian Gmeiner <christian.gmeiner@gmail.com>