drm/etnaviv: mmu: stop using iommu map/unmap functions

This is a preparation to remove the etnaviv dependency on the IOMMU
subsystem by importing the relevant parts of the iommu map/unamp
functions into the driver.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-By: Wladimir J. van der Laan <laanwj@gmail.com>