drm/amdgpu: add saved_bo to save vce 4.0 context when suspend

We are using PSP to resume firmware after suspend, and it is
resumed at where it got suspended, so we'd better save the
the context.

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