Claude review: drm/panthor: Replace cross-component register accesses with helpers

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

Patch Review

**Status: Good. Well-motivated.**

This patch eliminates cases where one component directly reads another component's registers. Key changes:

1. Moves `panthor_gpu_coherency_init()` from `panthor_device.c` to `panthor_gpu.c`:
```c
+void panthor_gpu_coherency_init(struct panthor_device *ptdev)
+{
+	gpu_write(ptdev->iomem, GPU_COHERENCY_PROTOCOL,
+		  ptdev->coherent ? GPU_COHERENCY_ACE : GPU_COHERENCY_NONE);
+}
```

2. Adds timestamp/cycle-count accessor helpers to avoid scheduler code reaching into GPU registers:
```c
+u64 panthor_gpu_get_timestamp(struct panthor_device *ptdev)
+{
+	return gpu_read64(ptdev->iomem, GPU_TIMESTAMP_LO);
+}
```

3. Adds a firmware doorbell helper:
```c
+void panthor_fw_ring_doorbell(struct panthor_device *ptdev, u32 doorbell)
+{
+	gpu_write(ptdev->iomem, MCU_DOORBELL(doorbell), 1);
+}
```

I verified the initialization ordering in `panthor_device.c`: `panthor_gpu_init()` is called at line 286 before `panthor_gpu_coherency_init()` at line 290, so `ptdev->gpu` (and its `iomem` field, added in patch 5) will be valid. Safe.

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Generated by Claude Code Patch Reviewer