main: Fix memleaks in mesa_use_program

Add freeing of SubroutineIndexes to the _mesa_free_shader_state.

Fixes: 4566aaaa ("mesa/subroutines: start adding per-context
subroutine index support (v1.1)")
Signed-off-by: Yevhenii Kolesnikov <yevhenii.kolesnikov@globallogic.com>
Reviewed-by: Marek Olšák <marek.olsak@amd.com>
(cherry picked from commit 882fe09a)