i965: fix tessellation regressions with gl_state_index16

Looks like one conversion was missed.

Fixes: e149a0253 (mesa,glsl,nir: reduce gl_state_index size to 2 bytes)
Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=105067
Signed-off-by: Dave Airlie <airlied@redhat.com>
Tested-by: Mark Janes <mark.a.janes@intel.com>
1 file changed