drm/etnaviv: flush shader L1 cache after user commandstream

The shader L1 cache is a writeback cache for shader loads/stores
and thus must be flushed before any BOs backing the shader buffers
are potentially freed.

Cc: stable@vger.kernel.org
Reviewed-by: Christian Gmeiner <cgmeiner@igalia.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>