aco: Initialize ds_state.front.writeMask.

Fix defect reported by Coverity Scan.

Uninitialized scalar variable (UNINIT)
uninit_use: Using uninitialized value ds_state.front. Field ds_state.front.writeMask is uninitialized.

Fixes: d488d0fd7ba ("aco: add framework for testing isel and integration tests")
Signed-off-by: Vinson Lee <vlee@freedesktop.org>
Reviewed-by: Rhys Perry <pendingchaos02@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9033>