drm/i915: Don't wake the device up to check if the engine is asleep

drm/i915: Don't wake the device up to check if the engine is asleep

If the entire device is powered off, we can safely assume that the
engine is also asleep (and idle).

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Fixes: a091d4ee931b ("drm/i915: Hold a wakeref for probing the ring registers")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Mika Kuoppala <mika.kuoppala@intel.com>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20180212093928.6005-1-chris@chris-wilson.co.uk
(cherry picked from commit 74d00d28a15c8452f65de0a9477b52d95639cc63)
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>