drm/i915: Hold rpm wakeref for printing the engine's register state

When dumping the engine, we print out the current register values. This
requires the rpm wakeref. If the device is alseep, we can assume the
engine is asleep (and the register state is uninteresting) so skip and
only acquire the rpm wakeref if the device is already awake.

Reported-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
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/20180212102415.24246-1-chris@chris-wilson.co.uk