drm/etnaviv: Fix the debug log of the etnaviv_iommu_map()

The value of the 'iova' variable is the start GPUVA that is going to be
mapped, its value doesn't changed when the mapping is on going.

Replace it with the 'da' variable, which is incremental and it reflects
the actual address being mapped exactly.

Signed-off-by: Sui Jingfeng <sui.jingfeng@linux.dev>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>