powerpc/pseries/ddw: simplify enable_ddw()

This drops rather useless ddw_enabled flag as direct_mapping implies
it anyway.

While at this, fix indents in enable_ddw().

This should not cause any behavioral change.

Signed-off-by: Alexey Kardashevskiy <aik@ozlabs.ru>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20211108040320.3857636-3-aik@ozlabs.ru