drm/selftests: missing error code in igt_buddy_alloc_smoke()

Set the error code to -ENOMEM if drm_random_order() fails.

Fixes: e6ff5ef8 ("drm/selftests: add drm buddy smoke testcase")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Reviewed-by: Arunpravin <Arunpravin.PaneerSelvam@amd.com>
Signed-off-by: Christian König <christian.koenig@amd.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220307125458.GA16710@kili