Claude review: drm: Fix DYNAMIC_DEBUG_MODULE propagation to subdirectories

[Claude Code Review Bot <claude-review@example.com>]

Patch Review

**Correctness: Good.** The two changes are:

```makefile
-CFLAGS-$(CONFIG_DRM_USE_DYNAMIC_DEBUG)	+= -DDYNAMIC_DEBUG_MODULE
+ccflags-$(CONFIG_DRM_USE_DYNAMIC_DEBUG)	+= -DDYNAMIC_DEBUG_MODULE
+subdir-ccflags-$(CONFIG_DRM_USE_DYNAMIC_DEBUG)	+= -DDYNAMIC_DEBUG_MODULE
```

1. `CFLAGS-y` is not a kbuild-recognized variable — this was simply dead code. Changing to `ccflags-y` is the correct fix (per `Documentation/kbuild/makefiles.rst:321`).

2. Adding `subdir-ccflags-y` propagates the flag to all subdirectories under `drivers/gpu/drm/`, which is essential since DRM drivers live in subdirectories like `amd/`, `i915/`, `nouveau/`, etc. Without this, files calling `drm_dbg()` and friends in those subdirectories would still get implicit-declaration errors for `_dynamic_func_call_cls`.

**Minor observation:** Per the kbuild documentation (`makefiles.rst:352-355`), `subdir-ccflags-y` applies to "*the kbuild file where they are present and all subdirectories*" — meaning it already covers the current directory. So the `ccflags-y` line is technically redundant; `subdir-ccflags-y` alone would suffice. This is harmless (duplicate `-D` flags for files directly in `drivers/gpu/drm/` don't cause problems), and being explicit about both scopes is a reasonable style choice. Not worth blocking over.

**Commit message:** Clear and accurate. The Fixes tag correctly references the commit that introduced the original `CFLAGS-y` typo. The note about `depends on BROKEN` is helpful context.

**Reviewed-by worthy:** Yes — this is a correct, minimal fix.

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