Claude review: drm/amdgpu: fix build for CONFIG_DRM_FBDEV_EMULATION=n

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

Patch Review

**Correctness: Good.** The stub returns `false`, which is semantically correct — if fbdev emulation is disabled, no GEM object can be the fbdev framebuffer. Both callers use the result in a `!drm_fb_helper_gem_is_fb(...)` guard to decide whether to unpin buffers:

```c
// amdgpu_display.c:1777
if (!drm_fb_helper_gem_is_fb(dev->fb_helper, fb->obj[0])) {
    struct amdgpu_bo *robj = gem_to_amdgpu_bo(fb->obj[0]);
    r = amdgpu_bo_reserve(robj, true);
```

```c
// radeon_device.c:1592
if (!drm_fb_helper_gem_is_fb(dev->fb_helper, fb->obj[0])) {
    struct radeon_bo *robj = gem_to_radeon_bo(fb->obj[0]);
    r = radeon_bo_reserve(robj, false);
```

With the stub returning `false`, the `!false` evaluates to `true`, meaning all buffers get unpinned when there's no fbdev — which is the right behavior since there's no kernel fbdev object to protect.

**Function signature: Matches exactly.** The stub's signature matches the real declaration at line 274-275:

```c
bool drm_fb_helper_gem_is_fb(const struct drm_fb_helper *fb_helper,
                             const struct drm_gem_object *obj);
```

**Placement: Correct.** The stub goes right after `#else` on line 276, inside the `#ifdef CONFIG_DRM_FBDEV_EMULATION` / `#else` / `#endif` structure.

**Fixes tag:** Points to the merge commit `02e778f12359` rather than either individual commit. This is appropriate since neither commit in isolation caused the breakage — it only manifested when both sides were merged together.

**Minor note on the subject line:** The subject says "drm/amdgpu" but this is really a fix to `include/drm/drm_fb_helper.h`, a core DRM header. Both amdgpu and radeon benefit from it. A more precise prefix might have been `drm/fbdev-helper`, but this is cosmetic and doesn't affect correctness.

**v2 note:** The changelog says v1 was revised per Thomas Zimmermann's feedback to restore only `drm_fb_helper_gem_is_fb()` (rather than presumably more stubs), which shows appropriate scope reduction.

**No issues found.** The patch is straightforward, correct, and properly scoped.

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