etnaviv: always do cpu_fini in transfer_unmap

The cpu_fini() call pushes the buffer back into the GPU domain, which needs
to be done for all buffers, not just the ones with CPU written content. The
etnaviv kernel driver currently doesn't validate this, but may start to do
so at a later point in time. If there is a temporary resource the fini needs
to happen before the RS uses this one as the source for the upload.

Also remove an invalid comment about flushing CPU caches, cpu_fini takes
care of everything involved in this.

Fixes: c9e8b49b885 ("etnaviv: gallium driver for Vivante GPUs")
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Reviewed-by: Philipp Zabel <p.zabel@pengutronix.de>
Reviewed-By: Wladimir J. van der Laan <laanwj@gmail.com>