drm/virtio: Fix return value for VIRTGPU_CONTEXT_PARAM_DEBUG_NAME

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: 7add80126bce ("drm/uapi: add explicit virtgpu context debug name")
Signed-off-by: Dmitry Osipenko <[email protected]>
Reviewed-by: Gurchetan Singh <[email protected]>
Link: https://patchwork.freedesktop.org/patch/msgid/[email protected]