anv: gem-stubs: return a valid fd got anv_gem_userptr()

Fixes invalid close(-1) in the unit tests.

Signed-off-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Cc: <mesa-stable@lists.freedesktop.org>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit da2d67fc)