drm/exynos/dma: Drop iommu_dma_init_domain() stub

drm/exynos/dma: Drop iommu_dma_init_domain() stub

Commit 1feda5eb77fc ("drm/exynos: Use selected dma_dev default iommu
domain instead of a fake one") removed the code around creating a
custom IOMMU domain, but forgot to remove the stub.

Remove the iommu_dma_init_domain() stub as the function is no longer
referenced, and was also made private to the IOMMU DMA code.

Signed-off-by: Chen-Yu Tsai <wenst@chromium.org>
Acked-by: Marek Szyprowski <m.szyprowski@samsung.com>
Signed-off-by: Inki Dae <inki.dae@samsung.com>