drm/i915/selftests: Use request managed wakerefs

Since commit 79ffac85 ("drm/i915: Invert the GEM wakeref
hierarchy"), the request creation itself took responsibility for
managing the engine/GT wakerefs and so we can remove the redundant grabs
in our selftests.

References: 79ffac85 ("drm/i915: Invert the GEM wakeref hierarchy")
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190620102432.31580-1-chris@chris-wilson.co.uk