x86, mm: Call is_untracked_pat_range() rather than is_ISA_range()

Checkin fd12a0d6

 made the PAT
untracked range a platform configurable, but missed on occurrence of
is_ISA_range() which still refers to PAT-untracked memory, and
therefore should be using the configurable.

Signed-off-by: H. Peter Anvin <hpa@zytor.com>
Cc: Jack Steiner <steiner@sgi.com>
Cc: Suresh Siddha <suresh.b.siddha@intel.com>
LKML-Reference: <20091119202341.GA4420@sgi.com>