drm/i915/selftests: add back the selftest() hook for the buddy

When we resurrected the selftest we forgot to add back the selftest()
hook, meaning the test is not currently run.

References: d1487389

 ("drm/i915/ttm Initialize the ttm device and memory managers")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Matthew Auld <matthew.auld@intel.com>
Cc: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Reviewed-by: Thomas Hellström <thomas.hellstrom@linux.intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210618133150.700375-1-matthew.auld@intel.com