Claude review: drm: uapi: Add macro for chipset specific event ID region

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

Patch Review

**Commit message:** Clear and well-written. It explains the motivation (elevating the comment-level contract to code) and references the v1 link. Good.

**`include/uapi/drm/drm.h` changes:**

```c
+#define DRM_EVENT_VENDOR_SPECIFIC(_v) ((_v) | 0x80000000)
```

Placed correctly right after `DRM_EVENT_CRTC_SEQUENCE` (the last generic event define) and before `struct drm_event_vblank`. The kerneldoc comment is appropriate for a UAPI header. The parenthesization of `_v` in `(_v)` and the outer parens are correct macro hygiene.

One nit: the name `DRM_EVENT_VENDOR_SPECIFIC` reads like a single event rather than a constructor/range macro. Names like `DRM_EVENT_CHIPSET` or `DRM_EVENT_DRIVER` might match the existing comment wording ("chipset specific") more closely, but this is subjective and the current name is perfectly clear in context.

**`include/uapi/drm/exynos_drm.h` changes:**

```c
-#define DRM_EXYNOS_G2D_EVENT		0x80000000
-#define DRM_EXYNOS_IPP_EVENT		0x80000002
+#define DRM_EXYNOS_G2D_EVENT		DRM_EVENT_VENDOR_SPECIFIC(0x0)
+#define DRM_EXYNOS_IPP_EVENT		DRM_EVENT_VENDOR_SPECIFIC(0x2)
```

Values preserved. `drm.h` is included at line 19 of this file. Correct.

**`include/uapi/drm/virtgpu_drm.h` changes:**

```c
-#define VIRTGPU_EVENT_FENCE_SIGNALED 0x90000000
+#define VIRTGPU_EVENT_FENCE_SIGNALED DRM_EVENT_VENDOR_SPECIFIC(0x10000000)
```

`0x10000000 | 0x80000000` = `0x90000000`. Value preserved. `drm.h` is included at line 27 of this file. Correct.

**`include/uapi/drm/vmwgfx_drm.h` changes:**

```c
-#define DRM_VMW_EVENT_FENCE_SIGNALED 0x80000000
+#define DRM_VMW_EVENT_FENCE_SIGNALED DRM_EVENT_VENDOR_SPECIFIC(0x0)
```

Value preserved. `drm.h` is included at line 32 of this file. Correct.

**No issues found.** This is a clean, correct, ABI-preserving patch.

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