etnaviv: Use reentrant screen lock around flush

etnaviv: Use reentrant screen lock around flush

The flush callback may be called on the same pipe context, and thus
the same stream, from two different threads of execution. However,
etna_cmd_stream_flush{,2}() must not be called on the same stream
from two different threads of execution as that would mess up the
etna_bo refcounting and likely have other ugly side effects.

Fix this by using a reentrant screen lock around the flush callback.

Signed-off-by: Marek Vasut <marex@denx.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Lucas Stach <l.stach@pengutronix.de>