host1x: debugfs_create_dir() can never return NULL

So there is no need to check for a value that can never happen.  No need
to check the return value all anyway, as any debugfs call can take the
result of this function as an option just fine.

Cc: Thierry Reding <thierry.reding@gmail.com>
Cc: dri-devel@lists.freedesktop.org
Cc: linux-tegra@vger.kernel.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>