drm/i915/selftests: Serialise nop reset with retirement

In order for the reset count to be accurate across our selftest, we need
to prevent the background retire worker from modifying our expected
state. To preserve the intent of symmetry, we apply this to both
i915_reset and i915_reset_engine, even though it strictly only affects
i915_reset_engine currently.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Mika Kuoppala <mika.kuoppala@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190626134433.6318-1-chris@chris-wilson.co.uk