hw/display/artist: Unbreak size mismatch memory accesses

Commit 5d971f9 ("memory: Revert "memory: accept mismatching sizes
in memory_region_access_valid") broke the artist driver in a way that
the dtwm window manager on HP-UX rendered wrong.

Fixes: 5d971f9 ("memory: Revert "memory: accept mismatching sizes in memory_region_access_valid")
Signed-off-by: Sven Schnelle <svens@stackframe.org>
Signed-off-by: Helge Deller <deller@gmx.de>

hw/display/artist.c

@@ -1237,20 +1237,16 @@ static const MemoryRegionOps artist_reg_ops = {
     .read = artist_reg_read,
     .write = artist_reg_write,
     .endianness = DEVICE_NATIVE_ENDIAN,
-    .valid = {
-        .min_access_size = 1,
-        .max_access_size = 4,
-    },
+    .impl.min_access_size = 1,
+    .impl.max_access_size = 4,
 };
 
 static const MemoryRegionOps artist_vram_ops = {
     .read = artist_vram_read,
     .write = artist_vram_write,
     .endianness = DEVICE_NATIVE_ENDIAN,
-    .valid = {
-        .min_access_size = 1,
-        .max_access_size = 4,
-    },
+    .impl.min_access_size = 1,
+    .impl.max_access_size = 4,
 };
 
 static void artist_draw_cursor(ARTISTState *s)