etnaviv: add fence fd support

Add etna_cmd_stream_flush2 with in-fence fd and out-fence fd support for
explicit fencing.

v3: added etna_cmd_stream_flush2 to etnaviv/etnaviv-symbol-check

v2: renamed etna_cmd_stream_flush_explicit to etna_cmd_stream_flush2

Signed-off-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-by: Eric Engestrom <eric.engestrom@imgtec.com>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>