drm/etnaviv: ensure write caches are flushed at end of user cmdstream

If the GPU is done with one user command stream the buffers referenced
by this command stream may go away and get unmapped from the MMU. If
the write caches are still dirty at this point later evictions will run
into MMU faults, killing the GPU.

Make sure the write caches are flushed before signaling completion
of the user command stream.

Signed-off-by: Lucas Stach <l.stach@pengutronix.de>