physmem: factor out direct access check into memory_region_supports_direct_access()

physmem: factor out direct access check into memory_region_supports_direct_access()

Let's factor the complete "directly accessible" check independent of
the "write" condition out so we can reuse it next.

We can now split up the checks RAM and ROMD check, so we really only check
for RAM DEVICE in case of RAM -- ROM DEVICE is neither RAM not RAM DEVICE.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250210084648.33798-4-david@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>