anv: Fix anv_descriptor_set reference error on deletion

anv: Fix anv_descriptor_set reference error on deletion

anv_descriptor_set_destroy uses the descriptor sets's set_layout member
to iterate the set's buffer views.  However, the set_layout reference
may have previously been freed.

On 64 bit builds, this bug generated valgrind errors but did not affect
CTS test results.  On 32 bit builds, it reliably produces assertions and
memory corruption.