Claude review: drm/msm/dp: rework HPD handling

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

Patch Review

This is the core patch (~600 lines changed). It drops the entire event queue/state machine and replaces it with a threaded IRQ handler that calls `drm_bridge_hpd_notify()` directly.

**Concern (potential bug):** The IRQ thread handler does not check `DP_DP_HPD_REPLUG_INT_MASK`:

```c
+	if (hpd_isr_status & DP_DP_HPD_UNPLUG_INT_MASK)
+		drm_bridge_hpd_notify(..., connector_status_disconnected);
+
+	if (hpd_isr_status & DP_DP_HPD_PLUG_INT_MASK)
+		drm_bridge_hpd_notify(..., connector_status_connected);
+
+	if (hpd_isr_status & DP_DP_IRQ_HPD_INT_MASK)
+		drm_bridge_hpd_notify(..., connector_status_connected);
```

The old code explicitly handled `DP_DP_HPD_REPLUG_INT_MASK` with an unplug+delayed plug sequence. The new code relies on `msm_dp_bridge_hpd_notify()` detecting replug via `msm_dp_aux_is_link_connected()` returning `ISR_HPD_REPLUG_COUNT`. But if the hardware sets only the REPLUG bit (without PLUG or UNPLUG), the IRQ thread will not call `drm_bridge_hpd_notify()` at all and the replug event is silently dropped. The old code previously queued explicit events for this. If hardware always sets PLUG alongside REPLUG this is fine, but that's a hardware assumption that should be documented.

**Observation:** The old replug handling had a 3-tick delay between unplug and plug events. The new `hpd_notify` handler does them back-to-back:

```c
+		if (hpd_link_status == ISR_HPD_REPLUG_COUNT) {
+			msm_dp_hpd_unplug_handle(dp);
+			msm_dp_hpd_plug_handle(dp);
```

This removes the debounce timing. If the display needs settling time between unplug and plug (which is common for DP), this could cause link training failures on replug events.

**Observation:** `msm_dp_snapshot()` loses its `event_mutex` protection. The function reads `dp->power_on` and accesses MMIO registers. Without locking, there's a TOCTOU race where power_on is true at the check but the device gets powered off before the register reads. This was pre-existing but the mutex removal makes it slightly worse.

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