Claude review: drm/i915/display: use drm backlight

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

Patch Review

**Status: Mostly good**

Clean integration following the same pattern as amdgpu.

**Concern**: The `Assisted-by: Sonnet:4` tag in the commit message should probably be `Assisted-by: Claude Sonnet 4 <noreply@anthropic.com>` or similar proper format, or dropped if the Intel maintainers don't want AI-assisted tags.

**Minor**: Same missing blank line issue as patch 8 between the backlight alloc block and the next if statement in `intel_dp_init_connector`.

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**Summary of critical issues to address before merge:**
1. Spinlock vs sleeping calls in `drm_backlight.c` — needs to be a mutex or refcounting deferred
2. Global property range clobbering across connectors — needs per-connector property instances
3. `drm_backlight_get_device` returns unprotected pointer — needs refcount or different API
4. lockdep assertions called outside lock in register/unregister
5. `WARN_ON` on brightness set failure is too aggressive for production
6. Resume backlight restore path may be broken after cache removal

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Generated by Claude Code Patch Reviewer