zink: use correct layer count when creating framebuffer

pipe_framebuffer_state::layers doesn't necessarily reflect the value we
need to use here, so we can use the util function to verify

Reviewed-by: Erik Faye-Lund <erik.faye-lund@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/6263>

src/gallium/drivers/zink/zink_context.c

@@ -494,7 +494,7 @@ create_framebuffer(struct zink_context *ctx)
 
    state.width = ctx->fb_state.width;
    state.height = ctx->fb_state.height;
-   state.layers = MAX2(ctx->fb_state.layers, 1);
+   state.layers = MAX2(util_framebuffer_get_num_layers(&ctx->fb_state), 1);
    state.samples = ctx->fb_state.samples;
 
    return zink_create_framebuffer(ctx, screen, &state);