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>

drivers/gpu/drm/tegra/gem.c

@@ -408,12 +408,9 @@ int tegra_bo_dumb_map_offset(struct drm_file *file, struct drm_device *drm,
 	struct drm_gem_object *gem;
 	struct tegra_bo *bo;
 
-	mutex_lock(&drm->struct_mutex);
-
 	gem = drm_gem_object_lookup(drm, file, handle);
 	if (!gem) {
 		dev_err(drm->dev, "failed to lookup GEM object\n");
-		mutex_unlock(&drm->struct_mutex);
 		return -EINVAL;
 	}
 
@@ -421,9 +418,7 @@ int tegra_bo_dumb_map_offset(struct drm_file *file, struct drm_device *drm,
 
 	*offset = drm_vma_node_offset_addr(&bo->gem.vma_node);
 
-	drm_gem_object_unreference(gem);
-
-	mutex_unlock(&drm->struct_mutex);
+	drm_gem_object_unreference_unlocked(gem);
 
 	return 0;
 }