drm/tegra: Use drm_gem_object_unreference_unlocked()

This only grabs the mutex when really needed, but still has a might-
acquire lockdep check to make sure that's always possible. With this
patch Tegra DRM is officially struct_mutex free, yay!

v2: refernce_unlocked doesn't exist as kbuild spotted.

Cc: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
[treding@nvidia.com: remove unused variables]
Signed-off-by: Thierry Reding <treding@nvidia.com>