riscv: Don't need to check the return value with NULL

The return value from pmap_l1_to_l2 and pmap_l2_to_l3 will never be
NULL, so don't need to check their return value with NULL.

Reviewed by: markj, mhorne
MFC after: 1 week
Pull Request: https://github.com/freebsd/freebsd-src/pull/1311