etnaviv: fix renderonly check in etna_resource_alloc
When the driver hasn't been initialized via renderonly, screen->ro will be NULL. This fixes a crash when passing USE_SCANOUT to etnaviv when it's missing renderonly. Signed-off-by:Simon Ser <contact@emersion.fr> Reviewed-by:
Christian Gmeiner <christian.gmeiner@gmail.com> Cc: mesa-stable Part-of: <mesa/mesa!12074>