drm/amdgpu: move WB_FREE to correct place

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: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Alex Deucher <alexander.deucher@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>