drm/rockchip: Set VOP for the DRM DMA device

Use VOP for DMA operations performed by DRM core. Rockchip DRM driver
is backed by a virtual device that isn't IOMMU-capable, while VOP is the
actual display controller device backed by IOMMU. Fixes "swiotlb buffer
is full" warning messages originated from GEM prime code paths.

Note, that backporting is non-trivial as this depends on
commit 143ec8d3f9396 ("drm/prime: Support dedicated DMA device for dma-buf
imports"), which landed in v6.16 and commit 421be3ee36a4 ("drm/rockchip:
Refactor IOMMU initialisation"), which landed in v5.19.

Reported-by: Daniel Stone <[e-mail]>
Fixes: 2048e3286f34 ("drm: rockchip: Add basic drm driver")
Cc: [e-mail] # v6.16+
Reviewed-by: Sebastian Reichel <[e-mail]>
Signed-off-by: Dmitry Osipenko <[e-mail]>
Tested-by: Cristian Ciocaltea <[e-mail]>
Signed-off-by: Heiko Stuebner <[e-mail]>
Link: https://lore.kernel.org/r/[e-mail]

drivers/gpu/drm/rockchip/rockchip_drm_drv.c

···
 		private->iommu_dev = ERR_PTR(-ENODEV);
 	else if (!private->iommu_dev)
 		private->iommu_dev = dev;
+
+	if (!IS_ERR(private->iommu_dev))
+		drm_dev_set_dma_dev(drm_dev, private->iommu_dev);
 }
 
 static int rockchip_drm_init_iommu(struct drm_device *drm_dev)