etnaviv: Fix double-free in etna_bo_cache_free()

The following situation can happen in a multithreaded OpenGL application.
A BO is submitted from etna_cmd_stream #1 with flags set for read.
A BO is submitted from etna_cmd_stream #2 with flags set for write.
This triggers a flush on stream #1 and clears the BO's current_stream
pointer. If at this point, stream #2 attempts to queue BO again, which
does happen, the BO will be added to the submit list twice. The Linux
kernel driver correctly detects this and warns about it with "BO at
index %u already on submit list" kernel message.

However, when cleaning the BO cache in etna_bo_cache_free(), the BO
which was submitted twice will also be free()d twice, this triggering
a glibc double free detector.

The fix is easy, even if the BO does not have current_stream set,
iterate over current streams' list of BOs before adding the BO to it
and verify that the BO is not yet there.

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Christian Gmeiner <christian.gmeiner@gmail.com>
Cc: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Christian Gmeiner <christian.gmeiner@gmail.com>
Reviewed-by: Wladimir J. van der Laan <laanwj@gmail.com>