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>

include/exec/memory.h

@@ -2995,10 +2995,13 @@ MemTxResult address_space_write_cached_slow(MemoryRegionCache *cache,
 int memory_access_size(MemoryRegion *mr, unsigned l, hwaddr addr);
 bool prepare_mmio_access(MemoryRegion *mr);
 
-static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
+static inline bool memory_region_supports_direct_access(MemoryRegion *mr)
 {
     /* ROM DEVICE regions only allow direct access if in ROMD mode. */
-    if (!memory_region_is_ram(mr) && !memory_region_is_romd(mr)) {
+    if (memory_region_is_romd(mr)) {
+        return true;
+    }
+    if (!memory_region_is_ram(mr)) {
         return false;
     }
     /*
@@ -3006,7 +3009,12 @@ static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
      * be MMIO and access using mempy can be wrong (e.g., using instructions not
      * intended for MMIO access). So we treat this as IO.
      */
-    if (memory_region_is_ram_device(mr)) {
+    return !memory_region_is_ram_device(mr);
+}
+
+static inline bool memory_access_is_direct(MemoryRegion *mr, bool is_write)
+{
+    if (!memory_region_supports_direct_access(mr)) {
         return false;
     }
     if (is_write) {