Claude review: drm/mediatek: Set dedicated DMA device and drop custom GEM callbacks

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

Patch Review

This is the big payoff patch, deleting 231 lines of custom GEM code (`mtk_gem.c` and `mtk_gem.h`). The key change is:

```c
-	for (i = 0; i < private->data->mmsys_dev_num; i++)
-		private->all_drm_private[i]->dma_dev = dma_dev;
+	drm_dev_set_dma_dev(drm, dma_dev);
```

The old code stored the DMA device in every `mtk_drm_private` instance. The new code calls the core API once on the DRM device. The driver ops are simplified to use the standard `DRM_GEM_DMA_DRIVER_OPS` macro, and the custom `mtk_gem_prime_import` / `mtk_gem_prime_import_sg_table` / `mtk_gem_dumb_create` are all dropped.

The commit message honestly documents the behavioral difference: the `VM_DONTDUMP` and `VM_IO` flags that were set by the custom `mtk_gem_object_mmap` will no longer be set by the standard `drm_gem_dma_mmap` helper. The reasoning that `VM_IO` doesn't make sense for `dma_alloc_attrs()` buffers is correct.

No issues. This is a clean simplification.

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