drm/i915/selftests: Fix live_workarounds to actually do resets

drm/i915/selftests: Fix live_workarounds to actually do resets

The test was missing some magic ingredients to actually trigger the
resets.

In case of the full reset we need the I915_RESET_HANDOFF flag set, and in
case of engine reset we need a busy request.

Thanks to Chris for helping with reset magic.

v2:
 * Grab RPM ref over reset.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20181130095211.23849-1-tvrtko.ursulin@linux.intel.com