drm/i915/selftests: Drop manual request wakerefs around hangcheck

We no longer need to manually acquire a wakeref for request emission, so
drop the redundant wakerefs, letting us test our wakeref handling more
precisely.

References: 79ffac85 ("drm/i915: Invert the GEM wakeref hierarchy")
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/20190626134433.6318-2-chris@chris-wilson.co.uk