Claude review: drm/hibmc: Use drm_atomic_helper_check_plane_state()

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

Patch Review

**Status: Good**

This replaces a hand-rolled plane state check with the standard `drm_atomic_helper_check_plane_state()` helper. The conversion is correct:

```c
ret = drm_atomic_helper_check_plane_state(new_plane_state, new_crtc_state,
                                          DRM_PLANE_NO_SCALING,
                                          DRM_PLANE_NO_SCALING,
                                          false, true);
```

- `can_position=false` (4th arg) correctly enforces that the plane must cover the full CRTC, preserving the original behavior where the plane had to match the display exactly.
- `can_update_disabled=true` (5th arg) allows check to proceed even with a disabled CRTC, which is cleaner than the old early return on `!crtc_state->enable`.
- Using `drm_atomic_get_new_crtc_state()` (read-only) instead of the old `drm_atomic_get_crtc_state()` (which adds/locks) is the correct modern pattern. The CRTC state is always already present at this point because `drm_atomic_helper_check_modeset()` runs before `drm_atomic_helper_check_planes()`.

The commit message notes that the visibility check is slightly relaxed (plane can now be *larger* than the CRTC, not just exactly equal). This is a correct behavioral change and well-documented.

The 128-byte pitch alignment check is preserved, which is important for hardware correctness.

No issues.

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