drm/virtio: use %llu format string form atomic64_t

The virtgpu driver prints the last_seq variable using the %ld or
%lu format string, which does not work correctly on all architectures
and causes this compiler warning on ARM:

drivers/gpu/drm/virtio/virtgpu_fence.c: In function 'virtio_timeline_value_str':
drivers/gpu/drm/virtio/virtgpu_fence.c:64:22: warning: format '%lu' expects argument of type 'long unsigned int', but argument 4 has type 'long long int' [-Wformat=]
  snprintf(str, size, "%lu", atomic64_read(&fence->drv->last_seq));
                      ^
drivers/gpu/drm/virtio/virtgpu_debugfs.c: In function 'virtio_gpu_debugfs_irq_info':
drivers/gpu/drm/virtio/virtgpu_debugfs.c:37:16: warning: format '%ld' expects argument of type 'long int', but argument 3 has type 'long long int' [-Wformat=]
  seq_printf(m, "fence %ld %lld\n",
                ^

In order to avoid the warnings, this changes the format strings to %llu
and adds a cast to u64, which makes it work the same way everywhere.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Dave Airlie <airlied@redhat.com>