anv: Initialize depth_bounds_test_enable when not explicitly set

anv: Initialize depth_bounds_test_enable when not explicitly set

This was causing uninitialized value to end up propagated to the
3DSTATE_DEPTH_BOUNDS packet, leading to asserts on packet
building due to the value being greater than 1.

Fixes: 939ddccb7a5 ("anv: Add support for depth bounds testing.")
Reviewed-by: Plamena Manolova <plamena.manolova@intel.com>
(cherry picked from commit 0aaf47f7cd50b9c07c1297fe8daff1b2693a6729)