drm/etnaviv: iommuv1: fold pagetable alloc and free into caller

Those functions are simple enough to fold them into the calling
function. This also fixes a correctness issue, as the alloc/free
functions didn't specifiy the device the memory was allocated for.

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>