drm/etnaviv: remove IOMMU dependency

Using the IOMMU API to manage the internal GPU MMU has been an
historical accident and it keeps getting in the way, as well as
entangling the driver with the inner workings of the IOMMU
subsystem.

Clean this up by removing the usage of iommu_domain, which is the
last piece linking etnaviv to the IOMMU subsystem.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>