winsys/amdgpu: build handles and flags list late on submit thread

winsys/amdgpu: build handles and flags list late on submit thread

This probably has only minor performance effects, but it simplifies some
subsequent code slightly.

Ideally, it could also be used to simplify the handling of slab buffers
in the same way, but unfortunately that's not possible as long as we need
indices for relocations.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>