Claude review: drm/vblank: timer: Estimate vblank timeout if timer is disabled

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

Patch Review

**Same clock domain issue as Patch 3:**
```c
*vblank_time = roundup(ktime_get_ns(), framedur_ns);
```

This returns a `CLOCK_MONOTONIC` value, while when the timer IS running (the `if (vblank->enabled)` branch), `*vblank_time` comes from `vtimer->timer.node.expires` which is now `CLOCK_BOOTTIME`. The caller receives timestamps from different clock domains depending on the code path. Should be `ktime_get_boottime_ns()` to match Patch 3's timer clock.

**Conceptual question:** The estimation assumes the frame-aligned phase established in `drm_crtc_vblank_start_timer` persists across timer disable/enable cycles. But since the alignment is computed fresh at start time (`roundup(now, framedur_ns)`), the phase will shift each time the timer is restarted. The "estimate" won't match the actual timer phase that was or will be active. This might be acceptable as a best-effort approximation, but the commit message's claim that "timeouts are aligned to frame duration, so we can round up to the next alignment" overstates the precision — the phase is arbitrary.

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