etnaviv: fix NULL pointer dereference in TS allocation

etnaviv: fix NULL pointer dereference in TS allocation

Commit c2b06e1a3820 ("etnaviv: add support for sharing the TS buffer")
introduced a problem similar to the one fixed in 3b3cd5128672 ("etnaviv:
fix renderonly check in etna_resource_alloc") in a different code path.
This causes a NULL pointer dereference when the screen is instanciated
on the render node. Fix it in the same way by just checking for a valid
pointer.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/19923>