KMSDRM_Vulkan_CreateSurface(): fixed pointer-cast warning on 32 bit

Closes: https://github.com/libsdl-org/SDL/issues/4284

src/video/kmsdrm/SDL_kmsdrmvulkan.c

@@ -198,7 +198,7 @@ SDL_bool KMSDRM_Vulkan_CreateSurface(_THIS,
     VkDisplaySurfaceCreateInfoKHR display_plane_surface_create_info;
 
     VkExtent2D image_size;
-    VkDisplayModeKHR display_mode = NULL;
+    VkDisplayModeKHR display_mode = (VkDisplayModeKHR)0;
     VkDisplayModePropertiesKHR display_mode_props = {0};
     VkDisplayModeParametersKHR new_mode_parameters = { {0, 0}, 0};