meson: Fix auto option for va

The same as the previous two patches, but for the libva state tracker.

Fixes: 724916c8
       ("meson: dedup gallium-xvmc logic")
Signed-off-by: Dylan Baker <dylan.c.baker@intel.com>
Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>
(cherry picked from commit 94cf3970)