drm/mediatek: Remove IOMMU_DMA select

We get a harmless build warning when trying to use the mediatek DRM
driver with IOMMU support disabled:

	warning: (DRM_MEDIATEK) selects IOMMU_DMA which has unmet direct dependencies (IOMMU_SUPPORT)

However, the IOMMU_DMA symbol is not meant to be used by drivers at all,
and this driver doesn't seem to have a strict dependency on it other
than using the mediatek IOMMU driver that does.

Since we also want to be able to do compile tests with the driver on
other platforms, the IOMMU_DMA symbol should not be selected here.

Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Philipp Zabel <p.zabel@pengutronix.de>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: http://patchwork.freedesktop.org/patch/msgid/1462997501-982363-1-git-send-email-arnd@arndb.de