rework etnaviv_buffer_queue(..)

With this change we should be more robust and are prepared
for flushing the iommu.

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
add etnaviv_cmd_mmu_flush(..) and adopt prefetch calculation

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>