genxml: add missing field values for 3DSTATE_SF

Fill out "Vertex Sub Pixel Precision Select" possible values.

CC: 18.3 19.0 <mesa-stable@lists.freedesktop.org>
Signed-off-by: Juan A. Suarez Romero <jasuarez@igalia.com>
Reviewed-by: Lionel Landwerlin <lionel.g.landwerlin@intel.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
(cherry picked from commit 3b423eeb)