drm/i915/gt: Use the local engine wakeref when checking RING registers

Now that we can atomically acquire the engine wakeref, make use of it
when check whether the RING registers are idle.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190812091045.29587-7-chris@chris-wilson.co.uk