etnaviv: Fix disabling of features

We must disable features before we fill our internal etna_specs
struct with etna_get_specs(..).

Fixes: d600b45ccc9 ("etnaviv: Switch to etna_core APIs")
Signed-off-by: Christian Gmeiner <cgmeiner@igalia.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/28757>