drm/mediatek: Stop using iommu_present()

Remove the pointless check. If an IOMMU is providing transparent DMA API
ops for any device(s) we care about, the DT code will have enforced the
appropriate probe ordering already. And if the IOMMU *is* entirely
absent, then attempting to go ahead with CMA and either suceeding or
failing decisively seems more useful than deferring forever.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patchwork.kernel.org/project/dri-devel/patch/fd1b62aa006556f29f37535814abfe41be63f7ae.1700746094.git.robin.murphy@arm.com/


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