drm/etnaviv: Drop the 'len' parameter of etnaviv_iommu_map() function

The 'len' parameter is the 4th argument, because it is not get used, so
drop it. No functional change.

Signed-off-by: Sui Jingfeng <suijingfeng@loongson.cn>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>