drm/amdgpu: wait for all rings to drain before runtime suspending

Add a safety check to runtime suspend to make sure all outstanding
fences have signaled before we suspend.  Doesn't fix any known issue.

We already do this via the fence driver suspend function, but we
just force completion rather than bailing.  This bails on runtime
suspend so we can try again later once the fences are signaled to
avoid missing any outstanding work.

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