drm/amdgpu: revert context to stop engine before mode2 reset

For some hang caused by slow tests, engine cannot be stopped which
may cause resume failure after reset. In this case, force halt
engine by reverting context addresses

Signed-off-by: Victor Zhao <Victor.Zhao@amd.com>
Acked-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>