drm/i915/selftests: Drop stale struct_mutex

A lately added test was missed when applying the struct_mutex removal
patches. Do so now.

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20191015085911.10317-1-chris@chris-wilson.co.uk