drm/mediatek: Add dma dev get function

This is a preparation for adding support for the ovl_adaptor sub driver
Ovl_adaptor is a DRM sub driver, which doesn't have dma dev. Add
dma_dev_get function for getting representative dma dev in ovl_adaptor.

Signed-off-by: Nancy.Lin <nancy.lin@mediatek.com>
Reviewed-by: AngeloGioachino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Tested-by: Bo-Chen Chen <rex-bc.chen@mediatek.com>
Link: https://patchwork.kernel.org/project/linux-mediatek/patch/20230321121859.2355-5-nancy.lin@mediatek.com/


Signed-off-by: Chun-Kuang Hu <chunkuang.hu@kernel.org>