drm/tegra: Fix lockup on a use of staging API

Commit bdd2f9cd ("Don't leak kernel pointer to userspace") added a
mutex around staging IOCTL's, some of those mutexes are taken twice.

Fixes: bdd2f9cd

 ("drm/tegra: Don't leak kernel pointer to userspace")
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Reviewed-by: Mikko Perttunen <mperttunen@nvidia.com>
Reviewed-by: Erik Faye-Lund <kusmabite@gmail.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/7b70a506a9d2355ea6ff19a8c4f4d726b67719b3.1497480754.git.digetx@gmail.com