drm/i915/selftests: Lock the drm_mm as we search

Hold onto the vm->mutex (to lock the drm_mm) to ensure that the rbtree
is stable as we search it for our scratch node.

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/20200128183458.3860022-1-chris@chris-wilson.co.uk