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: replaces the loop setting `private->all_drm_private[i]->dma_dev` and drops `mtk_gem.c` / `mtk_gem.h` entirely.

```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);
```

One observation: the old code set `.dma_dev` on **all** `mmsys_dev_num` drm_private instances, while the new code only calls `drm_dev_set_dma_dev()` on the single `drm` device. For multi-MMSYS configurations (e.g., MT8195 which has 2 MMSYS blocks), are there multiple `drm_device` instances that need the DMA device set? Looking at the code, all the `all_drm_private[i]` share the same `drm` device (they're all private data for sub-components bound to the same master), and `dma_dev` is stored in the private struct, not in `drm_device`. The new code sets it on `drm->dma_dev` which is the single DRM device, so this should be fine -- all paths now go through `drm_dev_dma_dev(drm)` which reads the single `drm->dma_dev` field. But it's worth confirming that none of the old `private->dma_dev` references remain elsewhere in the mediatek driver code beyond what's being removed.

The replacement of custom driver ops with `DRM_GEM_DMA_DRIVER_OPS` is clean:

```c
-	.dumb_create = mtk_gem_dumb_create,
+	DRM_GEM_DMA_DRIVER_OPS,
...
-	.gem_prime_import = mtk_gem_prime_import,
-	.gem_prime_import_sg_table = mtk_gem_prime_import_sg_table,
```

The commit message correctly notes the VM_DONTDUMP and VM_IO flag difference in the mmap path. The GEM DMA helpers don't set these because `dma_mmap_wc()` handles the mapping directly and the backing pages come from DMA-coherent allocations, making VM_IO inappropriate as the author notes.

**Minor**: No remaining references to `mtk_gem.h` in other mediatek files besides `mtk_crtc.c` (which is cleaned up)? The patch looks complete.

**Reviewed-by worthy**: Yes, with confirmation that multi-MMSYS setups work correctly with only one `drm_dev_set_dma_dev()` call.

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