[IA64] Fix DISCONTIGMEM without VIRTUAL_MEM_MAP

make allnoconfig currently fails to build because it selects DISCONTIGMEM
without VIRTUAL_MEM_MAP.  I see no particular reason this combination
ought to fail, so I fixed it by:

 - Including memory_model.h in all circumstances, except when both
   DISCONTIGMEM and VIRTUAL_MEM_MAP are enabled.
 - Defining ia64_pfn_valid() to 1 unless VIRTUAL_MEM_MAP is enabled

Signed-off-by: Matthew Wilcox <matthew@wil.cx>
Signed-off-by: Tony Luck <tony.luck@intel.com>