Claude review: drm/panthor: Use a local iomem base for GPU registers

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

Patch Review

**Status: Good.**

Adds `void __iomem *iomem` to `struct panthor_gpu`, initialized to `ptdev->iomem + GPU_CONTROL_BASE` (offset 0x0, so identical to `ptdev->iomem`):

```c
+	gpu->iomem = ptdev->iomem + GPU_CONTROL_BASE;
```

All GPU register accesses switch from `gpu_read(ptdev->iomem, GPU_XXX)` to `gpu_read(gpu->iomem, GPU_XXX)`. Since `GPU_CONTROL_BASE` is 0x0, the register defines don't need rebasing — they retain their existing values.

For IRQ, the GPU_INT registers are at offset 0x1000 (`GPU_INT_BASE`), so the IRQ iomem is set to:
```c
+	ret = panthor_request_gpu_irq(ptdev, ptdev->iomem + GPU_INT_BASE, ...);
```

The `GPU_INT_RAWSTAT`, `GPU_INT_CLEAR`, `GPU_INT_MASK`, `GPU_INT_STAT` defines are removed since they're now handled via the generic `INT_*` offsets relative to the IRQ's iomem base. Correct.

In `panthor_hw.c`, a local variable is introduced since `panthor_hw.c` doesn't have direct access to the `panthor_gpu` struct internals:
```c
+	void __iomem *gpu_iomem = ptdev->iomem + GPU_CONTROL_BASE;
```

This is clean.

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Generated by Claude Code Patch Reviewer