drm/etnaviv: iommuv1: fold pgtable_write into callers

A function doing a single assignment is not really helping the
code flow.

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