[Intel-gfx] Bug 95634, avoid fresh work on shutdown

[Chris Wilson]

In https://bugs.freedesktop.org/show_bug.cgi?id=95634 the cause of the
warning is that we populate the GGTT without waking the device. For the
purposes of suspend, if the device is already idle we have accomplished
our goal (of making the device idle). This series tries to avoid waking
the device simply to idle it (during suspend and elsewhere).
-Chris