p2m-pt: avoid hard-coding Xen's PAT

This makes the code much easier to understand.  No functional change
intended.  As per Andrew Cooper, the existing logic is questionable, but
this does not make it any worse.

Signed-off-by: Demi Marie Obenour <demi@invisiblethingslab.com>
Acked-by: Jan Beulich <jbeulich@suse.com>

xen/arch/x86/mm/p2m-pt.c

@@ -99,13 +99,13 @@ static unsigned long p2m_type_to_flags(const struct p2m_domain *p2m,
         return flags | P2M_BASE_FLAGS | _PAGE_RW | _PAGE_NX_BIT;
     case p2m_mmio_direct:
         if ( !rangeset_contains_singleton(mmio_ro_ranges, mfn_x(mfn)) )
-            flags |= _PAGE_RW;
+            flags |= _PAGE_RW | _PAGE_UCM;
         else
         {
-            flags |= _PAGE_PWT;
+            flags |= _PAGE_UC;
             ASSERT(!level);
         }
-        return flags | P2M_BASE_FLAGS | _PAGE_PCD;
+        return flags | P2M_BASE_FLAGS;
     }
 }