anv/device: initialize the list of enabled extensions properly

anv/device: initialize the list of enabled extensions properly

The loop goes through the list of enabled extensions marking them as
enabled in the list, but this relies on every other extension being
initialized to false by default.

This bug would make us, for example, advertise certain device extension
entry points as available even when the corresponding extensions had
not been enabled.

Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Fixes: abc62282b5c "anv: Add a per-device table of enabled extensions"
Cc: "18.0" <mesa-stable@lists.freedesktop.org>