drm/i915/selftests: Lock the drm_mm while modifying

Remember to lock the drm_mm as we modify it, lest it be modified in the
background by retire/free workers!

Signed-off-by: Chris Wilson <chris@chris-wilson.co.uk>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20190703091726.11690-2-chris@chris-wilson.co.uk