pmap_change_attr: Only fixup DMAP for DMAPed ranges

pmap_change_attr must change the memory type of both the requested KVA
and the corresponding DMAP mappings (if such mappings exist), to satisfy
an Intel requirement that two or more mappings to the same physical
pages must have the same memory type.

However, not all kernel mapped pages have corresponding DMAP mappings --
for example, 64-bit BARs. Skip fixing up the DMAP for out-of-bounds
addresses.

Submitted by: Steve Wahl <steve_wahl@dell.com>
Reviewed by: alc, jhb
Sponsored by: Dell Compellent
Differential Revision: https://reviews.freebsd.org/D4030