drm/virtio: Fix return value for VIRTGPU_CONTEXT_PARAM_DEBUG_NAME

The strncpy_from_user() returns number of copied bytes and not zero on
success. The non-zero return value of ioctl is treated as error. Return
zero on success instead of the number of copied bytes.

Fixes: 7add8012 ("drm/uapi: add explicit virtgpu context debug name")
Signed-off-by: Dmitry Osipenko <dmitry.osipenko@collabora.com>
Reviewed-by: Gurchetan Singh <gurchetansingh@chromium.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20231111224236.890431-1-dmitry.osipenko@collabora.com