drm/tegra: Don't take dev->struct_mutex in mmap offset ioctl

drm/tegra: Don't take dev->struct_mutex in mmap offset ioctl

Since David Herrmann's mmap vma manager rework we don't need to grab
dev->struct_mutex any more to prevent races when looking up the mmap
offset. Drop it and instead don't forget to use the unref_unlocked
variant (since the drm core still cares).

v2: Finally get rid of the copypasta from another commit in this
commit message. And convert to _unlocked like we need to (Patrik).

Cc: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Cc: Thierry Reding <thierry.reding@gmail.com>
Acked-by: Thierry Reding <thierry.reding@gmail.com>
Reviewed-by: Patrik Jakobsson <patrik.r.jakobsson@gmail.com>
Signed-off-by: Daniel Vetter <daniel.vetter@intel.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>