drm/i915/selftests: verify_gt_engine_wa() needs rpm wakeref

The mmio readback for verify_gt_engine_wa() also needs a runtime-pm
wakeref, so effectively do the entirety of both engine workarounds
tests. As such simplify the rpm behaviour here by acquiring the wakeref
for the whole of each subtest. It would be still useful to later verify
the registers retain their magic values across rpm suspend/resume.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Cc: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20181206180713.6827-1-chris@chris-wilson.co.uk