drm/etnaviv: initialize iommu domain page size

Since d16e0faa (iommu: Allow selecting page sizes per domain) the
iommu core demands the page size to be set per domain, otherwise any
mapping attempts will be dropped. Make sure to set a valid page size
for the etnaviv iommu.

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