drm/rockchip: Don't grab dev->struct_mutex for 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).

Aside: I stumbled over the mmap handler which directly does a
dma_mmap_attrs. But totally fails to grab a reference on the
underlying object and hence looks like it happily just leaks the ptes
since there's no guarantee the mmap isn't still around when
gem_free_object is called. Which the kerneldoc of dma_mmap_attrs
explicitly forbids.

v2: Fixup compile fail 0-day spotted.

Cc: Mark Yao <[e-mail]>
Reviewed-by: Thierry Reding <[e-mail]>
Signed-off-by: Daniel Vetter <[e-mail]>

drivers/gpu/drm/rockchip/rockchip_drm_gem.c

···
 	struct drm_gem_object *obj;
 	int ret;
 
-	mutex_lock(&dev->struct_mutex);
-
 	obj = drm_gem_object_lookup(dev, file_priv, handle);
 	if (!obj) {
 		DRM_ERROR("failed to lookup gem object.\n");
-		ret = -EINVAL;
-		goto unlock;
+		return -EINVAL;
 	}
 
 	ret = drm_gem_create_mmap_offset(obj);
···
 	DRM_DEBUG_KMS("offset = 0x%llx\n", *offset);
 
 out:
-	drm_gem_object_unreference(obj);
-unlock:
-	mutex_unlock(&dev->struct_mutex);
-	return ret;
+	drm_gem_object_unreference_unlocked(obj);
+
+	return 0;
 }
 
 /*