drm/amd/amdgpu: move drain_workqueue before shutdown is set

[background] when unloading amdgpu driver right after running a
workload, drain_workqueue is causing "Fence fallback timer
expired on ring sdma0.0". Under sriov, this issue will cause sriov
full access timeout and a reset happening.

move drain_workqueue before shutdown is set to allow ih process and
before enter full access under sriov to avoid full access time cost.

Signed-off-by: Victor Zhao <Victor.Zhao@amd.com>
Reviewed-by: Feifei Xu <Feifei.Xu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>