physmem: factor out RAM/ROMD check in memory_access_is_direct()

physmem: factor out RAM/ROMD check in memory_access_is_direct()

Let's factor more of the generic "is this directly accessible" check,
independent of the "write" condition out.

Note that the "!mr->rom_device" check in the write case essentially
disallows the memory_region_is_romd() condition again. Further note that
RAM DEVICE regions are also RAM regions, so we can check for RAM+ROMD
first.

This is a preparation for further changes.

Reviewed-by: Peter Xu <peterx@redhat.com>
Signed-off-by: David Hildenbrand <david@redhat.com>
Link: https://lore.kernel.org/r/20250210084648.33798-3-david@redhat.com
Signed-off-by: Peter Xu <peterx@redhat.com>