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 <[e-mail]>
Cc: Thierry Reding <[e-mail]>
Acked-by: Thierry Reding <[e-mail]>
Reviewed-by: Patrik Jakobsson <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>
Signed-off-by: Thierry Reding <[e-mail]>

drivers/gpu/drm/tegra/gem.c

···
 	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;
 	}
 
···
 
 	*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;
 }