drm/i915/selftests: Always lock the drm_mm around insert/remove

Be paranoid and make sure the drm_mm is locked whenever we insert/remove
our own nodes.

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/20191129095659.665381-1-chris@chris-wilson.co.uk