WB_FREE should be put after all engines's hw_fini
done, otherwise the invalid wptr/rptr_addr would still
be used by engines which trigger abnormal bugs.

This fixes couple DMAR reading error in host side for SRIOV
after guest kmd is unloaded.
Signed-off-by: NMonk Liu <Monk.Liu@amd.com>
Reviewed-by: NAlex Deucher <alexander.deucher@amd.com>
Signed-off-by: NAlex Deucher <alexander.deucher@amd.com>