drm/i915/selftests: Fixup atomic reset checking

We require that the intel_gpu_reset() was atomic, not the whole of
i915_reset() which is guarded by a mutex. However, we do require that
i915_reset_engine() is atomic for use from within the submission tasklet.

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/20190626134433.6318-3-chris@chris-wilson.co.uk