mm: cleanup is_highmem()

It is unnecessary to add CONFIG_HIGHMEM check in is_highmem(), which has
been done in is_highmem_idx(), and move is_highmem() close to
is_highmem_idx().  This has no functional impact.

Link: https://lkml.kernel.org/r/20220726131816.149075-1-wangkefeng.wang@huawei.com


Signed-off-by: Kefeng Wang <wangkefeng.wang@huawei.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>