drm/amdgpu: stop all rings before doing gpu recover

found recover_vram_from_shadow sometimes get executed
in paralle with SDMA scheduler, should stop all
schedulers before doing gpu reset/recover

Signed-off-by: Monk Liu <Monk.Liu@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Tested-by: Andrey Grodzovsky <andrey.grodzovsky@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>