drm/i915: Skip idling an idle engine

During suspend (or module unload), if we have never accessed the engine
(i.e. userspace never submitted a batch to it), the engine is idle. Then
we attempt to idle the engine by forcing it to the default context,
which actually means we submit a render batch to setup the golden
context state and then wait for it to complete. We can skip this
entirely as we know the engine is idle.

v2: Drop incorrect comment.

References: https://bugs.freedesktop.org/show_bug.cgi?id=95634
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Joonas Lahtinen <joonas.lahtinen@linux.intel.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1466776558-21516-1-git-send-email-chris@chris-wilson.co.uk