drm/etnaviv: drop GPU initialized property

Now that it is only used to track the driver internal state of
the MMU global and cmdbuf objects, we can get rid of this property
by making the free/finit functions of those objects safe to call
on an uninitialized object.

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