drm/i915: Do hw quiescing first during unload

If we force the hw to idle as our first step during unload, we can abort
the unload upon failure. Later we can probe whether the hardware remain
active even after we try to shut it down.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@ffwll.ch>