zink: verify geometry shader feature

We shouldn't try to use geometry shaders on implementations that don't
support them. So let's verify the feature before using it.

Fixes: 8028991f361 ("zink: enable gs pipe caps")
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7287>
1 file changed