Always Use Next Display Index

WIN_AddDisplay asserted that the returned index is the expected next
one in the list. This can be an unexpected number when displays are
add and removed in a batch (eg, Remote Desktop disconnect and reconnect).
Instead always just make use of the next index, even if it is not the
expected next index value. This fixes issue #9105

src/video/windows/SDL_windowsmodes.c

@@ -377,11 +377,10 @@ static void WIN_AddDisplay(_THIS, HMONITOR hMonitor, const MONITORINFOEXW *info,
     display.driverdata = displaydata;
     WIN_GetDisplayBounds(_this, &display, &displaydata->bounds);
     index = SDL_AddVideoDisplay(&display, send_event);
-    SDL_assert(index == *display_index);
     SDL_free(display.name);
 
 done:
-    *display_index += 1;
+    *display_index = index + 1;
 }
 
 typedef struct _WIN_AddDisplaysData