zink: stop setting nr_samples for null surfaces

this is a special value that shouldn't be set

Reviewed-by: Dave Airlie <airlied@redhat.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/12875>

src/gallium/drivers/zink/zink_surface.c

@@ -342,7 +342,7 @@ zink_surface_create_null(struct zink_context *ctx, enum pipe_texture_target targ
       return NULL;
 
    surf_templ.format = PIPE_FORMAT_R8_UINT;
-   surf_templ.nr_samples = samples;
+   surf_templ.nr_samples = 0;
    struct pipe_surface *psurf = ctx->base.create_surface(&ctx->base, pres, &surf_templ);
    pipe_resource_reference(&pres, NULL);
    return psurf;