drm/etnaviv: move flush_seq increment into etnaviv_iommu_map/unmap

The flush sequence is a marker that the page tables have been changed
and any affected TLBs need to be flushed. Move the flush_seq increment
a little further down the call stack to place it next to the actual
page table manipulation. Not functional change.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Tested-by: Guido Günther <agx@sigxcpu.org>
Acked-by: Guido Günther <agx@sigxcpu.org>