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).