mm/highmem: simplify is_highmem()

is_highmem() can be simplified by use of is_highmem_idx().  This patch
removes redundant code and will make it easier to maintain if the zone
policy is changed or a new zone is added.

(akpm: saves me 25 bytes of text per is_highmem() callsite)

Signed-off-by: Chanho Min <chanho.min@lge.com>
Reviewed-by: Dan Williams <dan.j.williams@intel.com>
Cc: Michal Hocko <mhocko@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>