cg3: fix up size parameter for memory_region_get_dirty()

The code was incorrectly calculating the end address rather than the size of
the required region.
Signed-off-by: NMark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: NGerd Hoffmann <kraxel@redhat.com>

hw/display/cg3.c

@@ -114,7 +114,7 @@ static void cg3_update_display(void *opaque)
         int update = s->full_update;
 
         page = y * width;
-        update |= memory_region_get_dirty(&s->vram_mem, page, page + width,
+        update |= memory_region_get_dirty(&s->vram_mem, page, width,
                                           DIRTY_MEMORY_VGA);
         if (update) {
             if (y_start < 0) {