anv: fix a cmd_buffer reference in simple shader
In utrace timestamp copy case cmd_buffer is NULL. Fixes: dbbcd5c3 ("anv: factor out generation kernel dispatch into helper") Signed-off-by:Tapani Pälli <tapani.palli@intel.com> Reviewed-by:
Lionel Landwerlin <lionel.g.landwerlin@intel.com> Part-of: <mesa/mesa!30475> (cherry picked from commit ff8953f6)