drbg: allow the ctr derivation function to be disabled in FIPS mode

drbg: allow the ctr derivation function to be disabled in FIPS mode

Word from the lab is:

    The use of the derivation function is optional if either an approved
    RBG or an entropy source provides full entropy output when entropy
    input is requested by the DRBG mechanism. Otherwise, the derivation
    function shall be used.

So our disallowing it's use was more than required.

Reviewed-by: Shane Lontis <shane.lontis@oracle.com>
(Merged from https://github.com/openssl/openssl/pull/16096)