powerpc/kuap: Fold kuep_is_disabled() into its only user

kuep_is_disabled() was introduced by commit 91bb3082 ("powerpc/32s:
Refactor update of user segment registers") but then all users but one
were removed by commit 526d4a4c ("powerpc/32s: Do kuep_lock() and
kuep_unlock() in assembly").

Fold kuep_is_disabled() into init_new_context() which is its only user.

Signed-off-by: Christophe Leroy <christophe.leroy@csgroup.eu>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/b2247147c0a8c830ac82966451647850df4a64da.1689091022.git.christophe.leroy@csgroup.eu