Claude review: drm/radeon: Test for fbdev GEM object with generic helper

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

Patch Review

**Overall**: Straightforward conversion. Removes radeon's `radeon_fbdev_robj_is_fb()` and replaces its call site with `drm_fb_helper_gem_is_fb()`.

**Correct conversion**: The original radeon code used `drm_gem_fb_get_obj(fb_helper->fb, 0)` directly, while the new helper covers this via the `else if (fb_helper->fb)` branch. The behavior is preserved.

**Same scoping improvement** as patch 1 — `robj` moves into the block where it's needed:

```c
-		robj = gem_to_radeon_bo(fb->obj[0]);
 		/* don't unpin kernel fb objects */
-		if (!radeon_fbdev_robj_is_fb(rdev, robj)) {
+		if (!drm_fb_helper_gem_is_fb(dev->fb_helper, fb->obj[0])) {
+			struct radeon_bo *robj = gem_to_radeon_bo(fb->obj[0]);
```

**Clean removal**: Both the implementation in `radeon_fbdev.c` and the declaration + static inline stub in `radeon_mode.h` are properly removed.

**Summary of actionable items**:
1. Change `EXPORT_SYMBOL_GPL` to `EXPORT_SYMBOL` for consistency with the rest of `drm_fb_helper.c`.
2. Fix typo: "test is" → "test if" in the kdoc comment.

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