drm/i915/selftests: Check that registers are preserved between virtual engines

Make sure that we copy across the registers from one engine to the next,
as we hop around a virtual engine.

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/20191010110252.17289-1-chris@chris-wilson.co.uk