drm/ttm: Add safety check for NULL man->bdev in ttm_resource_manager_usage

The `ttm_resource_manager_usage()` function currently assumes `man->bdev` is non-NULL when accessing `man->bdev->lru_lock`.
However, in scenarios where the resource manager is not fully initialized (e.g., APU platforms that lack dedicated VRAM, or incomplete manager setup),
 `man->bdev` may remain NULL. This leads to a NULL pointer dereference when attempting to acquire the `lru_lock`, triggering kernel OOPS.

Fix this by adding an explicit safety check for `man->bdev` before accessing its members:
- Use `WARN_ON_ONCE(!man->bdev)` to emit a one-time warning (a soft assertion) when `man->bdev` is NULL. This helps catch invalid usage patterns during debugging without breaking production workflows.
- Return 0 immediately if `man->bdev` is NULL, as a non-initialized manager cannot have valid resource usage to report.

Suggested-by: Christian König <[email protected]>
Suggested-by: Lijo Lazar <[email protected]>
Signed-off-by: Jesse Zhang <[email protected]>
Reviewed-by: Christian König <[email protected]>
Signed-off-by: Arunpravin Paneer Selvam <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

drivers/gpu/drm/ttm/ttm_resource.c

@@ -587,6 +587,9 @@ uint64_t ttm_resource_manager_usage(struct ttm_resource_manager *man)
 {
 	uint64_t usage;
 
+	if (WARN_ON_ONCE(!man->bdev))
+		return 0;
+
 	spin_lock(&man->bdev->lru_lock);
 	usage = man->usage;
 	spin_unlock(&man->bdev->lru_lock);