drm/sun4i: mixer: Call of_dma_configure if there's an IOMMU

The main DRM device is actually a virtual device so it doesn't have the
iommus property, which is instead on the DMA masters, in this case the
mixers.

Add a call to of_dma_configure with the mixers DT node but on the DRM
virtual device to configure it in the same way than the mixers.

Reviewed-by: Paul Kocialkowski <[e-mail]>
Signed-off-by: Maxime Ripard <[e-mail]>
Link: https://patchwork.freedesktop.org/patch/msgid/9a4da[e-mail]
(cherry picked from commit b718102dbdfd0285ad559687a30e27cc9124e592)
[Maxime: Applied to -fixes since it missed the merge window and display is
broken without it]
Signed-off-by: Maxime Ripard <[e-mail]>

drivers/gpu/drm/sun4i/sun8i_mixer.c

···
 	mixer->engine.ops = &sun8i_engine_ops;
 	mixer->engine.node = dev->of_node;
 
+	if (of_find_property(dev->of_node, "iommus", NULL)) {
+		/*
+		 * This assume we have the same DMA constraints for
+		 * all our the mixers in our pipeline. This sounds
+		 * bad, but it has always been the case for us, and
+		 * DRM doesn't do per-device allocation either, so we
+		 * would need to fix DRM first...
+		 */
+		ret = of_dma_configure(drm->dev, dev->of_node, true);
+		if (ret)
+			return ret;
+	}
+
 	/*
 	 * While this function can fail, we shouldn't do anything
 	 * if this happens. Some early DE2 DT entries don't provide