drm/sun4i: Use backend/mixer as dedicated DMA device

The sun4i DRM driver deals with DMA constraints in a peculiar way.
Instead of using the actual DMA device in various helpers, it justs
reconfigures the DMA constraints of the virtual display device using
the DMA device's device tree node by calling of_dma_configure().

Turns out of_dma_configure() should only be called from bus code.
Lately this also triggers a big warning through of_iommu_configure()
and ultimately __iommu_probe_device():

    late IOMMU probe at driver bind, something fishy here!

Now that the GEM DMA helpers have proper support for allocating
and mapping buffers with a dedicated DMA device, switch over to
it as the proper solution.

The mixer change was tested on a Pine H64 model B. The backend change
was only compile tested. Though I don't expect any issues, help testing
on an older device would be appreciated.

Acked-by: Jernej Skrabec <jernej.skrabec@gmail.com>
Link: https://patch.msgid.link/20260311094929.3393338-5-wenst@chromium.org
Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>

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