do something useful in etna_bo_cpu_prep(..) and etna_bo_cpu_fini(..)

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
call etna_bo_cpu_prep(..) at the end of switch_to_next_buffer(..)

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
move etna_cmd_stream_flush(...) call

Signed-off-by: Christian Gmeiner <christian.gmeiner@gmail.com>
fix etna_cmd_stream_flush(..) and etna_cmd_stream_finish(..)

etna_cmd_stream_flush(..) flushes the current stream and
switches to next buffer.

etna_cmd_stream_finish(..) flushes the current stream, waits
until it got processed by the GPU and switches to next
buffer.

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