drm/i915/selftests: Always use an active engine while resetting

drm/i915/selftests: Always use an active engine while resetting

Currently, we only try to reset a live engine for checking the whitelist
retention across a per-engine reset. For safety, it appears we need to
prime the system with a hanging spinner before performing a full-device
reset. (Figuring out the root cause behind the instability with handling
a reset during a no-op request is a challenge for another test, the
whitelist test has its own purpose.)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=109626
Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Link: https://patchwork.freedesktop.org/patch/msgid/20190213224805.32021-1-chris@chris-wilson.co.uk
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>