drm/amdgpu: Reverse the sequence of ctx_mgr_fini

drm/amdgpu: Reverse the sequence of ctx_mgr_fini
and vm_fini in amdgpu_driver_postclose_kms

csa buffer will be created per ctx, when ctx fini,
the csa buffer and va will be released. so need to
do ctx_mgr fin before vm fini.

Reviewed-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Junwei Zhang <Jerry.Zhang@amd.com>
Signed-off-by: Rex Zhu <Rex.Zhu@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>